Proof of concept implementation of an RBM in pure Python

Created by: twesterhout

First step towards using PyTorch modules with NetKet.

• to_json and from_json methods are dropped in favour of Save and Load methods. The fact that machines save their state in JSON format is now an implementation detail. Machines implemented in Python are free to use pickle for example.
• A "trampoline" class which enables sub-classing netket.machine.Machine from Python. Since this is still a work in progress, new features are not advertised in the docs.
• An example of a pure Python class is given which can be used with NetKet as a drop-in replacement for RbmSpin.