Rename the Learning section

Created by: gcarleo

The Learning section of the input, as it is, is too general, and should be restructured to take into account the future development of the code.

The best would be to group together all classes solving similar problems, either dealing to those problem in an exact way or with learning. The directory structure should reflect the input section. A first proposal is to remove the Learning section and just create a section GroundState for which as a method you can either specify FullEd, SparseEd, Sr, Gdesc and more and a section Dynamics for which as a method you can specify FullEd, SparseEd, (with possibly sub-fields to ask for imaginary-time dynamics), t-VMC (for the future) etc.

This would also allow to define specific sections corresponding, for example, to Supervised learning, Unsupervised, Tomography etc... once all these applications will be realized through the Challenges.

For backward compatibility in all versions v1.x.x, we will still need to support the Learning field though.