Relaxed composite

[bobmyhill]

This pull request:

• Adds a RelaxedComposite class, which extends Composite to equilibrate assemblages using only set_state(). The new class also supports calculation of partially and fully relaxed thermodynamic properties via user-defined isochemical reaction vectors.
• Adds a new example script, example_assemblage_relaxation.py, which demonstrates how to use the RelaxedComposite class to compute and compare thermodynamic properties (bulk modulus, thermal expansivity, heat capacity) for an olivine-wadsleyite assemblage under different relaxation scenarios.
• Adds new tests for the new class.

Other minor changes:

• Improved the check_eos_consistency function and related calculations to handle both minerals and composites.
• Clarified terminology and improved comments in the solution.py file, ensuring correct references to Gibbs energy (instead of Helmholtz) and updating documentation for derived properties.