Fix comments and docstrings in formal_integral.py

[KevinCawley]

Describe the bug
The docstrings and comments in the formal integral are bad. They do not follow the TARDIS guide to docstrings as of right now. There are several styles being displayed, along with incorrect information in the docstrings in regard to parameters and return types. Some comments also mention c code/extensions, which is outdated.

Variable names should be improved for comprehension to follow PEP8 and the TARDIS codestyle.
To Reproduce
See docstrings in the montecarlo/montecarlo_numba/formal_integral.py

Screenshots

System

• OS:

  • GNU/Linux
  • macOS

• Environment (conda list):
  tardis

Additional context