Use joblib as default parallel backend

[schlegelp]

With this PR we switch the default backend from pathos to joblib. pathos is still supported via the new backend parameter but joblibhas a few advantages:

1. In my hands it seeems considerable faster for small tasks (possibly due to lower overhead/faster serialization)
2. It temporarily keeps processes alive in the background to avoid having to spawn them again (this is a bit hit & miss)
3. Automatically determines batch sizes
4. Avoids oversubscription by automatically limiting the number of inner threads
5. Compatible with Dask (see also Hackathon: dask backend #111)

The plan is to let this incubate a little before merging.

TODO:

• explore using joblib for NBLAST
• make joblib a required dependency (currently optional)
• incorporate tqdm-joblib (it's only ~40 lines of code)