Submodule of QrmRf as drive channel

[PiergiorgioButtarini]

@PiergiorgioButtarini: in principle, we could make a minimal subclass making it clear that it is a qrm_rf used for this purpose.

However, I'd postpone after the Module reorganization, when the main class itself will be as minimal as possible.
In case you agree with the plan, just open an issue about this.

Originally posted by @alecandido in #800 (comment)

[alecandido]

Specific to a certain refactoring arc. Now outdated.