Add python documentation

Created by: gcarleo

As we move towards alpha/beta release for version 2.0, we need to have complete documentation for the new python functions.

The idea is to use the nice parser developed by @ooreilly here . This parser + some markdown templates will be used to automatically generate the files for the docs website of the site, here.

To do so, the python functions exposed through pybind11 should support a minimally modified version of the google-style docstrings.

A simple function with signature

operation(self: python_example.Operations, i: int, j: int, op_name: str='add') -> int

would have an associated docstring:

      A function performing one of the allowed operations between two integers.

      Args:
          i (int): The first parameter.
          j (int): The second parameter.
          op_name (str='add'): The type of operation.

      Returns:
          int: If op_name=='add' it perform the sum param1+param2, otherwise 0.

Help is welcome to add documentation following this style, once we finalize the parser bit.

One can add docstrings to pybind11 functions like this:

m.def("operation", &operation, R"EOF(
      A function performing one of the allowed operations between two integers.

      Args:
          i (int): The first parameter.
          j (int): The second parameter.
          op_name (str='add'): The type of operation.

      Returns:
          int: If op_name=='add' it perform the sum param1+param2, otherwise 0.
)EOF");