Multiscan tasks

A task to allow the fitting of mutiple scans should be written. This task should be able to launch multiple fits in parallel using the methods already implemented in #3 (closed).

Parallelism should be handled by mutliprocessing (see example in juno: https://gitlab.esrf.fr/loic.huder/juno/-/blob/e6902eafd1e250b15cfeb0291d7ee035f0e93d6f/juno/model/parallel_nxdata.py#L58)

Note that:

• mp_context shouldn't be needed (fork is fine in this context)
• max_workers should be carefully handled in a SLURM context: it can return the total number of cores rather than the number available by the SLURM reservation. See with @tvincent how to retrieve the correct number.

@denolf Feel free to correct/edit the above