…tion

## Added
- Added smooth cutoffs to distance tolerances
- Added smooth cutoffs to 3- and 4-body contributions to `evaluate_point()`
  - Viability function is now a more continuous surface (abrupt when transitioning below cutoff_min)
- Added descriptor generator procedure
- Added unit tests for distribution convergence
- Added probability density generation procedure
- Added `set_method_ratio_default` procedure

## Changed
- Changed Python docstrings style to match [NumPy Style Guide](https://numpydoc.readthedocs.io/en/latest/format.html)
- Specified expected types for Python arguments
- Converted all custom `modu` calls to `norm2`
- Optimised `evaluate` and `viability` modules
- Optimised array indexing
- Moved `atom_ignore_index` array to boolean inside `basis_type`
  - Acts as a mask for atoms

## Fixed
- Fixed handling of optional method ratio