Add support for reference frame information in UVData

[kartographer]

SMA uses doppler tracking at the time of observations to help keep spectral lines at fixed position, irrespective of the changing velocity of the observatory (either due to the rotation of the Earth or the Earth going around the Sun). For some observations (usually where you've got linedwidths that are ~< 10 km/sec), propagating this information is important, particularly if one needs to move between reference frames. We'd like to see this added under the banner of "optional variables" within UVData.