[LSST|dm-users 694] Minor changes to how daf_butlerUtils CameraMapper constructs its camera geometry object

[Russell Owen]

As part of fixing DM-3670 <https://jira.lsstcorp.org/browse/DM-3670> I changed a few details of how CameraMapper creates its camera geometry object. The two primary changes are:

1) If the policy provided to the CameraMapper constructor does not contain an entry named “camera” an exception is raised.

Formerly this would silently result in self.camera being set to None. This change is intended to avoid silent failures in making the camera geometry object. If you wish to create a camera mapper with no camera geometry, please override CameraMapper._makeCamera to return None.

The only breakage that this introduced was three unit tests in daf_butlerUtils itself. I fixed those by adding a small camera to that package in order to exercise CameraMapper._makeCamera.

2) Camera creation is delegated to a new method _makeCamera. This is intended to offer flexibility in how obs_* packages create cameras.

obs_* packages that relied on the default means of creating the camera object (all except obs_test) continue to work exactly as before. obs_test required a minor change.

— Russell