/**
@anchor DPSessionFileFormat

@page LEVEL2 DP Session File Format 


Since Trick 10, the DP Session file is changed to XML format. The Session XML Document 
Type Definitions(DTD) is defined as following:

@see @ref DataProducts "Data Products"

<center>@anchor session_dtd @b Session.dtd</center>

@verbatim

<!-- Trick DP Session definition -->
<!-- V 1.0 -->

<!ELEMENT tstart        (#PCDATA)>
<!ELEMENT tstop         (#PCDATA)>
<!ELEMENT frequency     (#PCDATA)>
<!ELEMENT file          (#PCDATA)>
<!ELEMENT dir           (#PCDATA)>
<!ELEMENT timename      (#PCDATA)>
<!ELEMENT run           (dir)>
<!ELEMENT product_files (file+)>
<!ELEMENT session       ((tstart|tstop|frequency)*, run+, product_files)>

<!ATTLIST run timename              CDATA #IMPLIED>

<!ATTLIST session version           CDATA #REQUIRED>

<!-- session@presentation attribute
     
     "simple"     - One curve per plot. #plots = #runs x #plot_specifications
     "comparison" - Multiple runs per plot. 
     "delta"      - difference of two runs.
     "contrast"   - some weird thing Danny dreamed up.
-->
<!ATTLIST session presentation      CDATA #REQUIRED>

<!-- session@mode attribute
     Indicates a restriction on what in a DP file.

     "plot"   - means that only the plots described in a DP file may be created.
     "table"  - means that only tables described in a DP file may be created.
      default - all plots and tables in the DP may be created.
-->
<!ATTLIST session mode              CDATA #IMPLIED>

<!ATTLIST session device            CDATA #IMPLIED>
<!ATTLIST session gnuplot_terminal  CDATA #IMPLIED>
<!ATTLIST session machine           CDATA #IMPLIED>
<!ATTLIST session port              CDATA #IMPLIED>
@endverbatim\n


The <b>@c session</b> element is required and is the root element of the Session XML file. 
It encloses all the other elements.

The optional <b>@c tstart, @c tstop</b> child elements of <b>@c session</b> specify the data 
within this range gets plotted. 

The optional <b>@c frequency </b> child element of <b>@c session</b> specifies the delta time 
between data points for display purposes. If it is smaller than the delta time of the recorded data 
or if it is not defined, then the recorded data frequency is used. If it is greater than the recorded 
data delta time, then it is used. It is specified in seconds.

The required <b>@c run</b> child element of @c session can occur once or more times. 
It specifies a simulation RUN_ directory as <b>@c dir</b> stated from which to retrieve data.

By default, the time name for each run is @c sys.exec.out.time, however, an optional attribute of each 
<b>@c run - @c timename</b> can be specified if a different name other than 
the default one is desired.

The required <b>@c product_files</b>  child element of <b>@c session</b> can only occur once. 
It specifies product specification file(s) to use for this session. Any number (but at least one) 
of product specification files may be specified through its <b>@c file</b> element. In other words, 
it can have one or more <b>@c file</b>  elements. In general the product specification files specify 
the type of product parameters to display for the product, and the display attributes for each parameter. 
Product specification files are discussed in greater detail in Section @ref DPProductFileFormat "6.2 DP Product File Format".

The required <b>@c version</b> attribute of <b>@c session</b> specifies the version of this file.

The required <b>@c presentation</b>  attribute of <b>@c session</b> is only useful when more than 
one data set is specified. It can be <b>@c Simple|Comparison|Delta|Contrast</b>. The <b>@c Simple</b>  
option will display the data products independently for all data sets specified. The <b>@c Comparison</b> option 
will display the data from all data sets in the same display. The <b>@c Delta</b> option subtracts 
the nth data set data from the first data set data and presents the result for data sets 2 through n in 
the same display. <b>@c Simple</b> is the default option.

@anchor session_device The <b>@c device</b> attribute of <b>@c session</b> specifies the visualization device 
for data output. By default, the output data is displayed on the user's current login terminal screen. 
Device types are currently <b>@c Terminal (default), @c Printer, and @c File</b>.
@li In order for Printer to work, you need to set your system variables as stated in @ref DPPlotPrinting "6.3 Plot Printing".

The <b>@c gnuplot_terminal</b> attribute of <b>@c session</b> instructs gnuplot to use the given 
terminal device for output. The terminals supported are <b>@c X11, @c postscript @c color, @c postscript, @c png, @c eps, and @c aqua
(@c X11 is the default)</b>. The “postscript” terminal yields black-n-white printable files. The @c png will create 
an image in Portable Network Graphics format. The @c aqua terminal is for Macintosh and uses native Aqua for plot display.

The optional <b>@c machine, @c port</b> attributes of <b>@c session</b> specify the name of a machine and the port
number for plotting.


@section LEVEL3 DP Session File Example

@verbatim
<?xml version="1.0" encoding="ISO-8859-1" standalone="no"?>
<!DOCTYPE session PUBLIC "-//Tricklab//DTD Session V1.0//EN" "Session.dtd">
<session device="Terminal" gnuplot_terminal="" mode="Plot" presentation="Simple" version="1.0">
    <tstart>-1.0E20</tstart>
    <tstop>1.0E20</tstop>
    <frequency>0.0</frequency>
    <run>
        <dir>/users/hchen/trick_sims/trunk/SIM_Ball++_L1/RUN_realtime</dir>
    </run>
    <run>
        <dir>/users/hchen/trick_sims/trunk/SIM_Ball++_L1/RUN_test</dir>
    </run>
    <product_files>
        <file>/users/hchen/trick_sims/trunk/SIM_Ball++_L1/DP_Product/DP_test_4.xml</file>
        <file>/users/hchen/trick_sims/trunk/SIM_Ball++_L1/DP_Product/DP_test_5.xml</file>
    </product_files>
</session>
@endverbatim

@see @ref session_dtd "Session.dtd"

*/