Add BatchLM fitter to perform many fits simultaneously

[ConnorStoneAstro]

Batching fits gives huge compute gains. The LM fitter for the batched version is slightly simplified to account for the fact that it is running many fits in parallel, and so should generally be used for simpler models. But that is exactly the use case for batching, many small simple models to be fit at once.

[Copilot]

Pull request overview

This PR introduces a batched Levenberg–Marquardt (LM) optimizer to efficiently fit many small, similar models/images in parallel, and documents how to use batched images and fitting in the tutorials.

Changes:

• Added BatchLM optimizer to run LM fits across a TargetImageBatch simultaneously.
• Extended LM functional utilities to support batch-wise NLL/gradient/Hessian and added batch_lm_step.
• Updated tutorial notebook to demonstrate creating TargetImageBatch and fitting it with BatchLM.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
docs/source/tutorials/ImageTypes.ipynb	Adds a new “Batch Images” tutorial section and a runnable BatchLM example.
astrophot/models/batch_model_object.py	Uses WCS fields from the windowed working_image for batched scene evaluation.
astrophot/fit/lm.py	Improves invalid-likelihood error messaging and relies on BaseOptimizer for max_iter.
astrophot/fit/func/lm.py	Updates NLL/linear algebra helpers for batch support; adds rho() and batch_lm_step().
astrophot/fit/func/init.py	Exposes batch_lm_step from the fit function namespace.
astrophot/fit/batch_lm.py	New BatchLM fitter implementation (forward/jacobian vmaps, batched LM stepping, uncertainty updates).
astrophot/fit/init.py	Exports BatchLM in the public astrophot.fit API.

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[codecov]

Codecov Report

❌ Patch coverage is 79.64072% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.17%. Comparing base (ac7b90a) to head (c83aff3).
⚠️ Report is 25 commits behind head on dev.

Files with missing lines	Patch %	Lines
astrophot/fit/batch_lm.py	76.52%	27 Missing ⚠️
astrophot/fit/func/lm.py	84.78%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #298      +/-   ##
==========================================
- Coverage   92.37%   91.17%   -1.21%     
==========================================
  Files         117      113       -4     
  Lines        5785     6199     +414     
==========================================
+ Hits         5344     5652     +308     
- Misses        441      547     +106     

Flag	Coverage Δ
unittests	91.17% <79.64%> (-1.21%)	⬇️

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