Nonlinear priors

[bobmyhill]

This PR adds the ability for users to add (potentially correlated) Gaussian priors to their parameters during nonlinear fitting.

Also included:

• Restructuring of the nonlinear fitting routines to use a model class derived from burnman.nonlinear_fitting.NonLinearModel.
• A small fix to the fit_eos example (the same fix was previously applied to the fitting tutorial).
• A new example illustrating the new functionality.
• A new test covering the new functionality.