Add proper ion & electron equilibration

[chris-ashe]

This pull request introduces significant improvements to the plasma power balance modeling and visualization, focusing on distinguishing between global and species (ion/electron) energy constraints, adding new user controls for confinement time ratios, and enhancing documentation and plotting of equilibration times.

Summary of most important changes:

Physics Model and Documentation Enhancements

• Expanded documentation in plasma_power_balance.md, plasma_confinement.md, and detailed_physics.md to clearly distinguish between global plasma power balance and species (ion/electron) power balances, including the mathematical formulations, solver recommendations, and new explanations for ion-electron equilibration time and power.

Input and Constraint Handling

• Added a new input variable f_t_fuel_ion_electron_energy_confinement to allow users to set the ratio of ion to electron energy confinement times, and updated the input validation to prevent simultaneous activation of conflicting power balance constraints (icc=2, icc=3, icc=4).

Visualization and Plotting Improvements

• Enhanced summary and detailed plasma parameter plots to display individual confinement times (τ_E, τ_e, τ_i), their ratios, and thermal energies for ions and electrons.
• Added new plots and text boxes for plasma equilibration times between electrons and deuterons, tritons, and alpha particles, both as profiles and volume-averaged values.
• Updated fusion power profile plots to use clearer variable names for alpha heating to electrons and ions.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.