Create ctypes only neighbor list API variant

This moves the lammps.get_neighlist() method to lammps.numpy.get_neighlist().
lammps.get_neighlist() now returns a NeighList object, while the NumPy variants
returns a NumPyNeighList object. The main difference between the two is that while
the ctypes variant returns neighlist elements as

atom idx (int), numneighs (int), neighbors (POINTER(c_int))

the NumPy variant returns

atom idx (int), neighbors (numpy.array)

doc/src/Python_module.rst

@@ -152,3 +152,7 @@ Classes representing internal objects
 .. autoclass:: lammps.NeighList
    :members:
    :no-undoc-members:
+
+.. autoclass:: lammps.NumPyNeighList
+   :members:
+   :no-undoc-members: