LSR vs SSB-based radial velocities for cat_entry

[attipaci]

In NOVAS (and hence SuperNOVAS), the radial velocity in cat_entry is implicitly defined relative to the Solar-System Barycenter. However, stellar velocities are often defined vs the Local Standard of Rest (LSR). NOVAS C, and prior SuperNOVAS releases had no way to distinguish the two. To provide better support for LSR velocities, while maintaining backward compatibility to the maximum extent, we could:

1. Document the API better to note that stellar radial velocities are defined relative to the SSB.
2. Provide a means to specify stellar (Galactic) sources with LSR velocities, such as with novas_lsr_to_ssb_vel() to convert LSR veclocities to SSB-based radial velocities for make_cat_entry().