[WIP] Add `freeze_profiles` option to fix the Z-average profiles

[bendudson]

Removes the DC, i.e. Z-averaged, component of the time-derivatives. This effectively adds artificial, time-varying, sources and sinks to keep the profiles fixed. Not recommended for nonlinear simulations, but useful for linear analysis.

[ZedThree]

Is this something that needs to be turned on consistently across components? If so, would it be better as a component itself, and just set something in the state?