spectral equivalencies from multiple conventions (work in progress)
relativistic equivalency implemented with example
implemented @astrofrog's comment: [convention]_velocity ->
doppler_[convention]
added doppler equivalencies to __all__
refactor equivalencies: include freq, wav, energy conversions for
doppler
fixed equivalencies issues, but tests won't run
added equivalency tests
added one more test.  I don't think I changed equivalencies...
tests fix
added new spectroscopic equivalency example
minor changes to formatting in new equivalencies example
clarification of equations
formatting change to equations
one last formatting change
added another example use showing `.to` in a different context
*sigh* if I'm going to define the conventions, I had best follow them...
implemented @iguananaut's suggestion: point to implementation
whitespace changes
added a CHANGES entry and updated the documentation to include a use
example
example->examples in docstrings