Ensure division is OK too, adapt tests, add line indicating API change
Added wave number equivalencies to spectral().
Expanded spectral_density() equivalencies to include PHOTLAM and PHOTNU. Added physical types for PHOTLAM and PHOTNU.
Updated relevant documentation.
Added text to CHANGES.rst as suggested by @eteq. Applied changes from astropy/astropy#1590 to this PR, as suggested by @astrofrog.