Feature parity for STEP simulations

[theo-brown]

In order to run STEP simulations in TORAX we need the following feature parity with RAPTOR and JETTO:

• Arbitrary prescribed profiles (Add support for prescribed source profiles #221 6aeb322)
• Bohm-gyro-Bohm transport (Add Bohm/gyro-Bohm transport model #266 Fix Bohm-gyro-Bohm model particle transport #406)
• Electron-cyclotron heating and current drive (Add EC heating and current drive #399 357f532)
• V-loop boundary condition (time derivative of psi_edge) ([WIP] Add Vloop BC #456)
• Confinement scaling of chi profiles
  • Bonus: pedestal imposed in chi rather than Te
• Basic neoclassical transport*
• $P_{rad}$ as a fraction of total heating, w/ prescribed radiation density profile*

Nice-to-haves:

• Ability to sum transport models*

[theo-brown]

Added three new features (marked *) from conversation with @fcasson.