Speed up PSF generation through caching

[wmwv]

Cache PSF

[X] 1. Cache at least for current ra, dec, band, pointing, sca.
[ ] 2. Cache for ra, dec, band, sca. Rotate WCS of PSF as necessary to match pointing.
[ ] 3. Cache base for sca, band. Calculate PSF for x, y from ra, dec. Rotate WCS of PSF as necessary to match pointing.

Moved from Roman-Supernova-PIT/phrosty#25
This functionality will likely get put into phrosty someday, but for now, the change is in diff-img.

[wmwv]

No, I totally confused myself. This is in phrosty.