Add ability to cache coordinate transformations within fit iterations

[jeremy-baier]

Profiling indicates that astrometric coordinate transformations are being called multiple times within the same iteration of a fit while the astrometry parameters are not changing. The result should be the same for all of those calls but they can sometimes be costly. Caching the result of the transformation until the astrometry parameters change could produce significant speed ups.

[dlakaplan]

This may relate to #1892 (which may be stalled)