Epoch propagation

[Markus Demleitner]

Dear DAL,

As the need to apply proper motions to our "big" catalogues becomes
more pressing with every year we move away from J2000.0, I think we
need to have standard ADQL user defined functions for epoch
propagation soon (more on this later).

To make the "good" way to apply these a bit simpler to have at least
for pg_sphere-using data centres, I've prepared a PR,
https://github.com/postgrespro/pgsphere/pull/8.  This will add
functions epoch_prop and epoch_prop_pos (modelled roughly after what
ESAC already has; cf. the capabilities of
https://gea.esac.esa.int/tap-server/tap).

I've temporarily put a build of the documentation on
http://docs.g-vo.org/pgsphere-beta/funcs.html#funcs.epochprop

I'd be extremely grateful for reviews or, indeed, any activity on the
PR in order to get it (or a fixed successor) merged in the foressable
future.

Thanks,

          Markus