Make output data from SimpleSolar and SolarPySAM consistent

[misi9170]

The return_outputs() methods of SimpleSolar and SolarPySAM both (correctly) return dictionaries, but they aren't consistent, which means that the two solar modules can't be exchanged directly. Currently, the return dictionary from SolarPySAM has keys "power_mw", "dni", and "aoi", whereas the SimpleSolar return dictionary contains "power" and "irradiance".

We should make these consistent (even if they hold null values in the SimpleSolar case) so that the modules can be readily interchanged.