Implement cross correlation API

[camposandro]

Implements the cross-correlation API. It includes a test case that computes the counts for the projected cross-correlation for two catalogs with weights (pcf_gals_weight, pcf_gals1_weight), as well as an end-to-end test that computes the estimate with the Davis-Peebles estimator. Closes #27.

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[github-actions]

Before [39e15e9]	After [f992e3e]	Ratio	Benchmark (Parameter)
2.90±1s	3.02±1s	1.04	benchmarks.time_computation
1.96k	864	0.44	benchmarks.mem_list

Click here to view all benchmarks.

[codecov]

Codecov Report

Attention: Patch coverage is 93.02326% with 3 lines in your changes missing coverage. Please review.

Files	Coverage Δ
src/corrgi/corrgi.py	100.00% <100.00%> (+8.33%)	⬆️
src/corrgi/estimators/estimator_factory.py	90.90% <100.00%> (+0.90%)	⬆️
src/corrgi/estimators/davis_peebles_estimator.py	92.85% <92.85%> (ø)
src/corrgi/estimators/estimator.py	96.00% <95.00%> (-0.78%)	⬇️
src/corrgi/estimators/natural_estimator.py	93.75% <50.00%> (-6.25%)	⬇️

... and 1 file with indirect coverage changes

[camposandro]

Davis-Peebles is the only estimator in Gundam for cross-correlation (the natural estimator is not available). #25 will allow us to implement Davis-Peebles and test e2e the cross-correlation pipeline - unblocking this PR.

[samotracio]

Code looks fine to me for the DP cross estimator