Add despike_jacobian option for adaptive resampling

[svank]

This adds an option to smooth over numerical quirkiness when, for example, the longitude wraps around in an all-sky map. The quirk occurs when computing the Jacobian, which is d(input-image pixel) / d(output-image pixel). This is done by finite differences, so when the input coordinates jump from one side of the map to the other, the Jacobian comes out extremely large, despite the actual transformation being unremarkable at that location. The new option looks for outlier Jacobian values, measured relative to a 3x3 local neighborhood, and replaces those outliers with a mean of the non-outlier values in the neighborhood.

Before this PR, the extremely-large Jacobian would cause the input-image region that is to be sampled to be extremely large---larger than a safety-valve threshold in the code that just outputs a nan for that output pixel. This caused a band of nans near the wrap-point's location in the output image. With this change, that seam is handled correctly.

Looks like this might close #195.

starfield_sample, _ = reproject.reproject_adaptive(
    (all_sky_map, all_sky_wcs), data_wcs, data.shape,
    x_cyclic=True, despike_jacobian=False)
starfield_sample_despiked, _ = reproject.reproject_adaptive(
    (all_sky_map, all_sky_wcs), data_wcs, data.shape,
    x_cyclic=True, despike_jacobian=True)

plt.figure(figsize=(10,5))
plt.subplot(121, projection=data_wcs)
plt.imshow(np.isnan(starfield_sample))
plt.title("np.isnan(starfield_sample)")

plt.subplot(122, projection=data_wcs)
plt.imshow(np.isnan(starfield_sample_despiked))
plt.title("np.isnan(starfield_sample_despiked)")

[codecov]

Codecov Report

Merging #366 (4ab3b3a) into main (8ea36e7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #366   +/-   ##
=======================================
  Coverage   92.50%   92.50%           
=======================================
  Files          24       24           
  Lines         840      840           
=======================================
  Hits          777      777           
  Misses         63       63           

Impacted Files	Coverage Δ
reproject/adaptive/core.py	97.77% <ø> (ø)
reproject/adaptive/high_level.py	100.00% <ø> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[astrofrog]

Looks great, thanks!