NetKet V2.0b1

Created by: gcarleo

Summary for version 2.0

This PR introduces the first beta version for NetKet 2.0.

Major changes in version 2.0

1. NetKet now fully exposes its internal types and classes, thus becoming a full-fledged python library built on a monolithic c++ core
2. Python Bindings are provided using pybind11, thus fully addressing issue #8 (closed)
3. As a result of this transformation, JSON input is not accepted anymore and the netket executable does not exist anymore
4. A great deal of flexibility in the usage of NetKet comes with these changes, removing the majority of constraints coming from the JSON-based input and usage pattern
5. Bindings are provided for the great majority of NetKet classes, with the exception of some internals, that don't need python exposure.

Reasons behind these changes

1. The most common current usage pattern of NetKet is not to write directly the JSON file, but use the python scripts we have in the Tutorials/ folder. That's why moving to a python library is a desirable goal, improving the user experience and opening the way to a number of applications previously hard or impossible to perform
2. With a full-fledged python library it is also much easier to submit multiple jobs with different parameters, and have more flexibility on the output
3. There is a substantial overhead for maintaining the JSON support. Supporting all the possible ways of putting together the NetKet classes, in a consistent way, does not scale well given the increasing size of this project.
4. It becomes easier to interface to a whole spectrum of Python-only libraries, including advanced visualization tools and state-of-the-art machine learning libraries (pytorch, tensorflow etc)
5. New generations of students (unfortunately?) are not very familiar with c++, and strive to have a pure Python library

Installing, Tutorials, Examples

To install the latest development version do

pip install . 

The directory /Tutorials/ contains pynotebook tutorials. More will be added. Several example codes showcasing a specific application are contained in /Examples.

Other remarks

• Version 2.0 removes all "glue" classes, such as Graph,Hamiltonian etc. The corresponding abstract interfaces remain, with little changes.

• We introduce the concept of Operator, which generalizes Hamiltonian,Observable etc. Hamiltonian and Observable disappear.

• We introduce a LocalOperator, and all associated overloaded operations, for example

hi=nk.Spin(s=0.5,graph=g)
X=[[0,1],[1,0]]
o=nk.LocalOperator(hi,X,[0])*(nk.LocalOperator(hi,X,[1]))

defines a quantum operator X(0)*X(1), automatically performing the tensor product. Other operations such as product times a scalar and addition of local operators are also supported.