Redundant method eval_filter_grid in ImageSource

[chris-langfield]

Address #38

I'm not sure if this method is unnecessary. The code in eval_filter_grid does some manipulation with the filter_indices of the source that is only applicable within that class.

It is only used in two implementations of the Estimator.compute_kernel method: in aspire.reconstruction.mean and aspire.covariance.covar

[codecov]

Codecov Report

Merging #517 (d7ccd1a) into develop (b146d33) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #517   +/-   ##
========================================
  Coverage    86.54%   86.54%           
========================================
  Files          106      106           
  Lines         7660     7660           
========================================
  Hits          6629     6629           
  Misses        1031     1031           

Impacted Files	Coverage Δ
src/aspire/operators/__init__.py	100.00% <ø> (ø)
src/aspire/source/image.py	95.40% <ø> (-0.21%)	⬇️
src/aspire/covariance/covar.py	88.79% <100.00%> (+0.09%)	⬆️
src/aspire/operators/filters.py	96.33% <100.00%> (+0.22%)	⬆️
src/aspire/reconstruction/mean.py	100.00% <100.00%> (ø)

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[chris-langfield]

As discussed earlier it seems like the best solution here is to simply move the whole method into operators.filters as a module method (outside of the Filter class).

[garrettwrong]

One question, one todo, otherwise lgtm. Hit those and should be ready to undraft. Thanks

[chris-langfield]

Rebased / squash prior to undraft