Copied write_tsi_model_to_netcdf to irradiance directory (from tsi directory).  Began adding modifications to alter how we define 'time' as a netcdf attribute.

Wrote Robodoc header to nrl2_to_irradiance. Added function call to write_tsi_model_to_netcdf.
Comments in code
Modified write_tsi_model_to_netcdf to use new structure variable for tsi = totirrad