feat: Add minifit fitter to optimize on downsampled target

[ConnorStoneAstro]

Often an image contains far more pixels than are actually needed to optimize parameters (at least for initial parameter fitting). This fitter takes a downsampled version of the target and applies an optimizer on it.

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (73e4109) 79.15% compared to head (97e9a44) 79.31%. Report is 22 commits behind head on dev.

Files	Patch %	Lines
astrophot/__init__.py	50.00%	4 Missing :warning:
astrophot/models/model_object.py	85.71%	3 Missing :warning:
astrophot/image/window_object.py	95.91%	2 Missing :warning:
astrophot/fit/minifit.py	96.87%	1 Missing :warning:
astrophot/image/image_object.py	92.85%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #155 +/- ## ========================================== + Coverage 79.15% 79.31% +0.16% ========================================== Files 86 87 +1 Lines 7186 7252 +66 ========================================== + Hits 5688 5752 +64 - Misses 1498 1500 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.