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:

• [x] Arbitrary prescribed profiles (#221 https://github.com/google-deepmind/torax/commit/6aeb3223223fecce9af1d0d8dcd90a4ac624851a)
• [x] Bohm-gyro-Bohm transport (#266 #406)
• [x] Electron-cyclotron heating and current drive (#399 https://github.com/google-deepmind/torax/commit/357f53234adcf2aef8b4b317796e406bc0738221)
• [ ] V-loop boundary condition (time derivative of psi_edge) (#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.