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		N/A	#221
Bohm-gyro-Bohm transport	✅	#452	#266, #406
Electron-cyclotron heating and current drive	✅		#399
V-loop boundary condition (time derivative of ψₑ)	⏳	N/A	#456
Confinement scaling of chi profiles	❌	#533
Pedestal imposed in chi rather than Te	❌
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*

[theo-brown]

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