adapt to redesigned colorterms in meas_astrom (DM-378)

Instead of being a global, colorterms are now stored in a Config, and
can be retrieved by astrometry_net_data version.
config: remove meas_mosaic filtermap for y

When using a Pan-STARRS catalog (default for the HSC survey), we have
y-band reference measurements, so don't want to calibrate to z-band.
When using SDSS, this will need to be re-instated, e.g., through a
command-line override.
Merge branch 'tickets/DM-378a'