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)
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'.
