Improving and expanding the Photometry class

[lzkelley]

Currently the Photometry class contains only simple primitives, with different types of flux/brightness measures enumerated. This is something that could be both streamlined and expanded significantly. This issue is a place to record ideas and brainstorm.

• Streamlining: for example, by using Quantity classes which include errors and units internally
• Expanding:
  • Using a new, more general 'brightness'/'intensity' class which can readily convert between different flux/magnitude units.
  • Allowing conversion between flux and luminosity using the distance
  • convert from bands to bolometric and visa-versa using more detailed modeling.