dcor: distance correlation and energy statistics in Python

• Fixes wrong types in u_distance_stats_sqr.
• Add missing types in rowwise.

• New documentation theme.
• Added links in the theory.
• Added examples to the documentation.
• Warning added to partial distance correlation/covariance docstrings by @jltorrecilla in https://github.com/vnmabus/dcor/pull/47

• Improve the computation time of distances for Numpy arrays, which improves performance for energy distance and the naive case of distance covariance/correlation.
• Improve AVL algorithm for distance covariance performance to bring it closer to mergesort.
• Refactor distance covariance to be able to compute distance correlation without additional calls to the covariance function.

• @jltorrecilla made their first contribution in https://github.com/vnmabus/dcor/pull/47

Full Changelog: https://github.com/vnmabus/dcor/compare/0.5.7...0.6