Refactor LammpsNumpyWrapper to numpy_wrapper

LammpsNumpyWrapper was a class that was defined inside of the
lammps.numpy property when it was first accessed. This made it hard
to document the methods of this class.

This commit extracts this utility class into the lammps module and
renames it to 'numpy_wrapper'.

doc/src/Python_module.rst

@@ -44,6 +44,9 @@ functions. Below is a detailed documentation of the API.
 .. autoclass:: lammps.lammps
    :members:
 
+.. autoclass:: lammps.numpy_wrapper
+   :members:
+
 ----------
 
 The ``PyLammps`` class API