Refactor cython code

[lbluque]

Summary

Move cython code to main package and renamed to correlations.pyx.

This is in order to make eventual pypi packaging more organized.

Functions now need to be imported as

from smol.correlations import corr_from_occupancy

Checklist

• Code is in the standard Python style.
  Run pycodestyle and flake8
  on your local machine.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Tests have been added for any new functionality or bug fixes.
• All existing tests pass.