AST-backed BoundedFields (e.g. TransformBoundedField) perform much better on
arrays of points, than single operations. This change gives about a 17x
speedup for `photoCalib.instFluxToNanojansky(src, 'slot_PsfFlux')` for a
TransformBoundedField-based photoCalib. It is about 25% slower for spatially-
constant PhotoCalibs but still below a millisecond, so that difference
should not matter.

This code block had already turned up twice, and I'll need it again for
`calibrateCatalog`, so pull it out into a private method.

Refactor catalog tests to make them safer.