Perfect nested sampling functionality

[williamjameshandley]

Description

A set of scripts for generating perfect nested sampling runs which will prove useful in developing techniques such as importance sampling #132.

Changes:

1. Implemented a spherical gaussian as found in John Skilling's original paper
2. Implemented a correlated gaussian, which can be used to simulate cosmological likelihoods
3. Moved the wedding cake class into a function in the same framework as the two above

Checklist:

• [X] I have performed a self-review of my own code
• [X] My code is PEP8 compliant (flake8 anesthetic tests)
• [X] My code contains compliant docstrings (pydocstyle --convention=numpy anesthetic)
• [X] New and existing unit tests pass locally with my changes (python -m pytest)
• [X] I have added tests that prove my fix is effective or that my feature works