Handle more WhereWhen time co-ordinate systems.

[timstaley]

Currently we blindly accept the following co-ord system tags, and assume they are similar enough not to warrant conversion:

vp.definitions.sky_coord_system.utc_fk5_geo,
vp.definitions.sky_coord_system.utc_fk5_topo,
vp.definitions.sky_coord_system.utc_icrs_geo,
vp.definitions.sky_coord_system.utc_icrs_topo,

Essentially, if the timestamp isn't UTC, we ignore the WhereWhen section and drop the 'coords' table entry.
It shouldn't be too hard to implement conversion from all the time-system options (GPS, TDB, TT) to UTC using the astropy routines. Likewise it may be worth converting from FK5 to ICRS where appropriate for the sake of logical consistency, although the actual difference is small (<80mas).
We currently do not store a 'geo/topo/bary' (reference frame centre) field. Might consider adding this in future.