refactored optimal thermobarometry

[bobmyhill]

This pull request modularizes the code for thermobarometric inversion in burnman.tools.thermobarometry, extracting reusable functions for reaction matrix construction, assemblage state setting, and affinity misfit calculation from the estimate_conditions function.

Summary of changes:

• Added get_reaction_matrix, assemblage_set_state_from_params, and assemblage_affinity_misfit functions to encapsulate core steps in thermobarometric inversion, improving code reuse and clarity.
• Refactored the main estimate_conditions function to use these new utilities, streamline the optimization logic, and support optional priors on pressure and temperature via condition_priors and condition_covariances arguments.
• Updated the example script to import and utilize the new modular functions, and added a diagnostic plot showing how the reduced chi-squared misfit varies with compositional degrees of freedom, aiding in model interpretation and debugging.