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, gpmixing.py or something similar, along with dependencies and any necessary versions of packages (see Issue Assess changes to external dependence versions #140 as well concerning major package versions)
• 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. Making the code general and not needing to directly edit source code will be the hurdle to overcome.

[asemposki]

Timeline update: this will need to be done by the BAND v1.0.0 release next spring/summer, so I will begin working on this after the Taweret v1.2.0 (and BAND v0.5) release.

[asemposki]

Working on this currently on the add-gp-bmm branch.