The astrometry/photometry name passed to _do_load_refcat_and_fit() used to be
capitalized, but that changed long ago. Either way, I should have made the
comparisons against the name use .lower() to catch both cases. This should
fix a few bugs that are not properly exercised by the jointcal tests.

The reference selectors were being set by the caller; these lines were just
wrong (it should have been self.X, not self.config.X)!

Review comments suggested clarifications in config validation, since
colorterms are only applied for photometry.