DM-12315: Generalize ap_pipe to non-HiTS data

[kfindeisen]

This PR moves most of the config defaults out of the source code (see lsst-dm/legacy-ap_verify_hits2015#9, lsst/obs_decam#88, and lsst/obs_subaru#143). Note that, depending on the templates or refcats being used, runs of ap_pipe are much more likely to need a user config file.

This PR also makes some minor maintenance updates to ap_pipe.

[mrawls]

It looks like you've moved all this refcat filterMap business into config/apPipe.py in ap_verify_hits2015. Do all our "datasets" now need to include a config like this in order to run ap_pipe on them? Could this information live in the obs_package's config/apPipe.py instead?

[kfindeisen]

The obs packages already have defaults (in processCcd.py) for lsst-dev:/datasets/refcats/htm/ps1_pv3_3pi_20170110. However, we use different refcats in the datasets (in particular, Gaia).

I think that if we're providing dataset-specific refcats, then we should provide the corresponding config in the dataset as a matter of self-consistency. Otherwise, changes to the refcats assumed by the obs package could break the dataset (though it may only be obvious if we're not running on lsst-dev).