gh-212: metadata for bias computation

[JaimeRZP]

Adds more metadata to the maps' fields used to compute the Jacknife covariance matrix.
The history of the branch seems linked to my previous PR??

[ntessore]

Try git checkout more_metadata && git rebase --onto main FootprintFilter more_metadata && git push -f?

[JaimeRZP]

Try git checkout more_metadata && git rebase --onto main FootprintFilter more_metadata && git push -f?

Nice git arcane magic!

[ntessore]

The Weights class doesn't have the extra metadata yet. We should also

1. figure out better names for the metadata, particularly variance, which isn't the variance;
2. figure out whether to store density and effective density separately.

Let's discuss offline.

[ntessore]

Excellent work 🎉