added doppler equivalencies to main class
moved doppler stuff to wcs
moved equivalencies to specwcs
refactor: doppler moved out of basespectrum, equivalencies rearranged
removed reference frequency from specwcs.  I actually disagree with
doing this; now the reference frequency can only be set once, upon
reading.  There needs to be a way to modify it on the fly, and at the
moment the only way to do that is reset the equivalencies completely and
add a new doppler one... I guess I'll just have to write wrappers for
that
Merge pull request #40 from keflavich/reference_frequency

Add doppler convention & reference frequency for velocity conversion