Adding the Engle 2023/2024 XUV and gyrochronology models

[sengle01]

Proposed changes

Added new models for XUV calculations, and magnetic braking

Types of changes

All modifications are in stellar.c and stellar.h

The XUV models are pretty smoothly integrated

The braking models were integrated with their own 'empirical' variable because I wasn't exactly sure how to disable other calculations that were also influencing the rotations. Rory has figured it out on his end, so there will be a cleaner way to integrate the braking models.