Add deep convection to gcm-driven SCM setup

[costachris]

Add relaxation timescale for deep convection in gcm-driven SCM setup

[szy21]

The parameters change looks good to me. Gabriele can take a look at the code change.

[Sbozzolo]

Can you turn forcing_type into a julia type and use dispatch instead of matching strings?

[Sbozzolo]

Code looks good (altough, I think that it is more efficient to dispatch over the first argument). Could you please add some documentation, docstrings, and expand upon what the config external_forcing_type is?

Your help message for external_forcing_type is "External forcing type", which doesn't teach much

[szy21]

Actually, I think we should add these time scales to parameters instead of having a type. No need to change it now, but whoever uses it or changes it again for other types (e.g. reanalysis) should use parameters instead I think.