Cosmo-utils

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- Changes to the function `~cosmo_utils.utils.stats_funcs.Stats_one_arr`.
Fixed issue with `bin_statval` argument within the function.

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- Changes to `~cosmo_utils.utils.work_paths.get_output_path`. Now all the
catalogues are under the `external` directory.

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- Changes to `~cosmo_utils.utils.work_paths.get_output_path`. Now all the
catalogues are under the `external` directory.

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- Removed `pairwise_counting` module (Cython version) from the main package

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- Modified the function `~cosmo_utils.ml.ml_utils.train_test_dataset` to accept
`pandas.DataFrame` input variables.

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- Added new feature ``sigma_clf_c`` which corresponds to the scatter in
``log(L)`` for central galaxies in the `conditional luminosity function`.