gh-289: notebook for Gaussian covariance

[JaimeRZP]

This PR adds an example notebook that computes the covariance of a Gaussian random field with the Heracles tool set.

Fixes: #289
Fixes: #286

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
py3.10	82.74% <100.00%> (+0.02%)	⬆️
py3.11	82.74% <100.00%> (+0.02%)	⬆️
py3.12	82.70% <100.00%> (+0.02%)	⬆️
py3.13	82.40% <100.00%> (+0.02%)	⬆️
py3.9	82.77% <100.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
heracles/healpy.py	96.47% <100.00%> (+0.12%)	⬆️
heracles/mapper.py	100.00% <100.00%> (ø)

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[ntessore]

@JaimeRZP let's make these full-sky Gaussian random fields with a mock spectrum, and work on the resulting maps directly, so that we have a theory covariance to compare to.

Oh, I think the notebooks are confusing the file previewer, I see on my machine that the new notebook doesn't have a catalogue or a window!