OCSADV-154: Remove support for FK4/B1950

This PR removes support for the FK4/B1950 coordinate system. It includes the following changes:

- B1950 is removed as a constant (in several places) and all special handling that switches on J2000 vs B1950 is collapsed into a single code path.
- Code is added in `To2015B` to precess existing targets in B1950 forward in time to J2000 on import. Required math code is copied into the migration package to allow future deletion of the original code in `jsky`. There are associated tests, one of which is failing (probably bad data; this will be resolved prior to merging).

Following this merge J2000 will be the only supported coordinate system, and a follow-on PR will remove the associated enumerations and so on, which now have only a single member.

:-1: don't merge yet; figure out why the `sig Oct` test is failing