Analytic models etc

Summary of changes:
* Added analytic model classes that uses `astropy.modeling` and related tests (spacetelescope/pysynphot#5)
* Replaced `OverlapError` with `PartialOverlap` (spacetelescope/pysynphot#47)
* Moved flux conversion function from `spectrum.py` back to `units.py` now that circular import is not an issue
* Wavelength conversion now completely uses `astropy.units` `spectral()`, including wave number.
* Flux conversion now mostly uses `astropy.units` `spectral_density()`, except magnitudes and counts.
* C-extension is now documented properly

4 tests are currently failing because of `astropy.modeling` API changes that have not been finalized (astropy/astropy#1680).