Add doppler convention & reference frequency for velocity conversion

This PR builds on the `from_fits` work @wkerzendorf did yesterday by adding a doppler equivalency as a necessary component of the WCS (in some sense, it is, but in case users don't care, there should be a sensible default).

One tricky bit now is that there are SOME FITS conventions that specify what the doppler convention is, but I don't think they're standardized.  For now, the capability is in place...