Optional: Add an option to only tune one part of the model

[PhilippBach]

Hi @OliverSchacht ,

just as an idea... maybe it makes sense to optionally let users provide separate tuning times for the different learners, e.g., through a tune_time dictoinary that maps the tuning time to each of the nuisance components ml_m, ml_l etc.

I think, it's not necessarily the case that the tuning time should be split equally across these tasks

[OliverSchacht]

Yes, maybe the parameter time should be renamed to tune_time and should be int, dict or None, with dict being a dictionary of the nuisance components.

[PhilippBach]

I've been playing around with it a bit and will add a commit in the next hour...