Improve fitting functions

[bobmyhill]

General

• Add priors to fitting variables.

fit_phase_proportions_to_bulk_composition

• Add cross-referencing documentation linking to optimize.linear_fitting.weighted_constrained_least_squares()
• Add documentation explaining how residuals and covariances are calculated

fit_composition_to_solution

• Add cross-referencing documentation linking to optimize.linear_fitting.weighted_constrained_least_squares()
• Add documentation explaining how residuals and covariances are calculated

[bobmyhill]

Priors added in #643
Docs added in #644