Adding GP mixing to Taweret as a mixing module

[asemposki]

I will be adding the option of model mixing via GPs to Taweret, which will require a whole new mixing module to implement. This will mean I need to complete the following tasks:

• Adding a new mixing module file, gps.py or something similar
• Full doc strings for the code in the module
• Make a Jupyter Book tutorial notebook for new users

The new mixing module will be based off of the GP class in scikit-learn, and will need to pull from scikit-learn to be used, which I foresee being the main potential difficulty of building this method.