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:

Feature	Status	Outstanding issues	PRs
Arbitrary prescribed profiles	✅		#221
Bohm-gyro-Bohm transport	✅	#452	#266, #406
Electron-cyclotron heating and current drive	✅		#399
V-loop boundary condition (time derivative of ψₑ)	✅		#456
Basic neoclassical transport*	❌	#530
$P_{rad}$ as a fraction of total heating, w/ prescribed radiation density profile*	✅		#572

Nice-to-haves:

• Ability to sum transport models*
• Confinement scaling of chi profiles (Implement transport coefficient feedback (or new state equation) for desired H-factor #533)
• Pedestal imposed in chi rather than Te

[theo-brown]

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

[theo-brown]

With #456 brought in, we're only really missing neoclassical transport now! The reason for particularly needing it is that in order to do scenario optimisation we need self-consistency, which means that prescribing an inner patch isn't really sufficient.