Default to only compute median frame for 1st batch

[taylorbell57]

Before this PR, a different median frame was computed for each batch. For those like me who always only a single huge batch, this bug was absent. However, if you used more than one batch (e.g. had too little RAM to load all segments at once), then sometimes things like the computed centroid or curvature correction would change between different segments which would cause jump discontinuities between segments

This partially resolves Issue #538, although I still need to add an override option to the ECF

[codecov-commenter]

Codecov Report

Attention: 324 lines in your changes are missing coverage. Please review.

Comparison is base (559f703) 57.02% compared to head (27a752a) 57.63%. Report is 148 commits behind head on main.

Files	Patch %	Lines
src/eureka/S5_lightcurve_fitting/models/GPModel.py	5.00%	152 Missing :warning:
src/eureka/S5_lightcurve_fitting/s5_fit.py	30.30%	23 Missing :warning:
src/eureka/S3_data_reduction/nirspec.py	28.00%	18 Missing :warning:
src/eureka/S3_data_reduction/niriss_python.py	11.76%	15 Missing :warning:
src/eureka/S5_lightcurve_fitting/plots_s5.py	71.15%	15 Missing :warning:
src/eureka/S5_lightcurve_fitting/likelihood.py	54.83%	14 Missing :warning:
src/eureka/S3_data_reduction/s3_reduce.py	79.68%	13 Missing :warning:
src/eureka/S3_data_reduction/nircam.py	61.90%	8 Missing :warning:
...curve_fitting/differentiable_models/PyMC3Models.py	71.42%	8 Missing :warning:
src/eureka/S5_lightcurve_fitting/fitters.py	76.47%	8 Missing :warning:
... and 18 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #582 +/- ## ========================================== + Coverage 57.02% 57.63% +0.61% ========================================== Files 94 96 +2 Lines 11282 11557 +275 ========================================== + Hits 6433 6661 +228 - Misses 4849 4896 +47 ```

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