Save center_ephemeris_time to the SAR sensor model

The center_ephemeris_time should be saved in the SAR sensor model, or else the SAR model will fail to be loaded, when the following code is encountered: 

https://github.com/USGS-Astrogeology/usgscsm/blob/da685687d4842d4f60d0a427348288517237664b/src/UsgsAstroSarSensorModel.cpp#L62