Used the mods field in the attributes to indicate of "--" was specified in the header file. We
still save the units as "1" to keep it compatible with udunits. When outputting the variable
in data recording or variable server we check to see if the mods field for "--" is set. We
output "--" if the mods field is set. Also allowed "--" to persist in data products.
refs #254
Initial checkin of using udunits for measurement units conversions. Replaced
our conversion code in ICG and swig code to use udunits.
Also added code to convert unit symbols that we used in Trick that are different
names in udunits.
Added a sim_object that will eventually allow users to specify a custom
set of files to read units information.
refs #231
