Add support for hookup and antpos files

This adds support for arbitrary correlator hookups to make the system
antenna-to-input agnostic.  It also includes "real" antenna positions in
the MIRIAD datasets rather than dummy placeholder coordinates (A,A,A)
for antenna A.  See the comments in cn_rx.py for details about how the
antenna positions are stored (units, frame, etc.).

This relies on having the following two
configuration mapping files (from the "papercfg" repository) installed:

/etc/papercfg/psa128/ant_to_pos.yml - antenna to position
/etc/papercfg/psa128/fxin_to_antpol.yml - correlator input to antpol

On paper1, the papercfg repository is cloned into "/home/obs/papercfg".
Move relabel logic earlier in filewrite_callback

This moves the relabeling logic to the very beginning of
filewrite_callback.  This is needed because the relabeling need to
happen as early as possible in that function, *before* any of the
antenna or polarization info are used.

This also ensures that the baselines being written are in the proper
order (i.e. a1<=a2) (and cross-polarization XY vs YX).