fixes a small bug that prevented changes in the comment symbol on writing

closes #790
change name of parameter back to comment
Merge pull request #1167 from hamogu/fix790

fixes a small bug that prevented changes in the comment symbol on writing
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