Chapter on parallelism

[GaelVaroquaux]

The high-level parallel libraries have matured so much that it is now possible to do fairly easy parallel computing. Now is probably the time to add a chapter on this in scikit-learn.

I would see the following sections:

1. joblib
2. dask
3. concurrent.futures

[GaelVaroquaux]

If someone picks this one up, I'm sure that @mrocklin would be happy to help on the dask part.

[souravsingh]

I would be willing to help on this. Should I start with joblib first?

[rootsmusic]

dpnp is a parallel extension for numpy

[stefanv]

I think we should probably stick with stock standard, community maintained packages.