Move counting routines to estimator class

[camposandro]

The correlation counts depend on the estimator selected. This pull request creates the Estimator abstract class and adds the counting routine as its method. Currently there is just the implementation for the natural estimator (DD/RR - 1). The remaining ones will be added according to #23. Closes #24.

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[github-actions]

Before [a21b9aa]	After [a6448ea]	Ratio	Benchmark (Parameter)
4.15±0.5s	2.57±0.9s	~0.62	benchmarks.time_computation
3.94k	2.1k	0.53	benchmarks.mem_list

Click here to view all benchmarks.

[codecov]

Codecov Report

Attention: Patch coverage is 89.33333% with 8 lines in your changes missing coverage. Please review.

Files	Coverage Δ
src/corrgi/correlation/angular_correlation.py	100.00% <100.00%> (ø)
src/corrgi/correlation/projected_correlation.py	100.00% <100.00%> (ø)
src/corrgi/corrgi.py	91.66% <100.00%> (-2.09%)	⬇️
src/corrgi/dask.py	72.97% <ø> (-2.64%)	⬇️
src/corrgi/estimators/natural_estimator.py	100.00% <100.00%> (ø)
src/corrgi/correlation/correlation.py	87.75% <80.00%> (-0.89%)	⬇️
src/corrgi/estimators/estimator.py	96.77% <96.77%> (ø)
src/corrgi/estimators/estimator_factory.py	90.00% <90.00%> (ø)
src/corrgi/correlation/redshift_correlation.py	0.00% <0.00%> (ø)