PhotonDCR config bugs

[jmeyers314]

Placing here so I don't forget:

Looks like PhotonDCR._opt_params is missing an entry for obj_coord.

Also, there's a typo: zenit_coord -> zenith_coord.

Need to add some config usage tests.

[rmjarvis]

One comment about this. The zenit_coord is of course a typo-bug, which apparently isn't covered by unit tests. But obj_coord is handled by the PhotonDCRBuilder here: https://github.com/GalSim-developers/GalSim/blob/releases/2.3/galsim/config/photon_ops.py#L220

[rmjarvis]

Done via #1160