Output "--" if that was specified in the header file.

Unit tests failed sometimes.  Found that an attributes structure for time
in the data recording class was unintialized.  This led to the mods
field to have random data, and sometimes triggering sys.exec.out.time
to be assigned no units.  Fixed it by zeroing out the structure.

refs #254

trick_source/sim_services/DataRecord/DataRecordGroup.cpp

@@ -60,6 +60,7 @@ Trick::DataRecordGroup::DataRecordGroup( std::string in_name ) :
  freq(DR_Always),
  start(0.0) ,
  cycle(0.1) ,
+ time_value_attr() ,
  num_variable_names(0),
  variable_names(NULL),
  variable_alias(NULL),