Additional spectral density equivalencies

This implements some of the spectral density equivalencies described in #440. I/We still need to add conversion to/from ``photons s^-1 [area^-1] spectralunit^-1``, and optionally including conversions where the spectral unit is energy rather than wavelength or frequency.

@eteq - feel free to add some of these if you want, as I probably won't be able to do it this week. I don't think it's 0.2 crucial though.