add an example for looking up and looping over a neighbor list

doc/src/Python_neighbor.rst

@@ -1,6 +1,43 @@
 Neighbor list access
 ====================
 
+Access to neighbor lists is handled through a couple of wrapper classes
+that allows to treat it like either a python list or a NumPy array.  The
+access procedure is similar to that of the C-library interface: use one
+of the "find" functions to look up the index of the neighbor list in the
+global table of neighbor lists and then get access to the neigbor list
+data. The code sample below demonstrates reading the neighbor list data
+using the NumPy access method.
+
+.. code-block:: python
+
+   from lammps import lammps
+   import numpy as np
+
+   lmp = lammps()
+   lmp.commands_string("""
+   region box block -2 2 -2 2 -2 2
+   lattice fcc 1.0
+   create_box 1 box
+   create_atoms 1 box
+   mass 1 1.0
+   pair_style lj/cut 2.5
+   pair_coeff 1 1 1.0 1.0
+   run 0 post no""")
+
+   # look up the neighbor list
+   nlidx = lmp.find_pair_neighlist('lj/cut')
+   nl = lmp.numpy.get_neighlist(nlidx)
+   tags = lmp.extract_atom('id')
+   print("half neighbor list with {} entries".format(nl.size))
+   # print neighbor list contents
+   for i in range(0,nl.size):
+       idx, nlist  = nl.get(i)
+       print("\natom {} with ID {} has {} neighbors:".format(idx,tags[idx],nlist.size))
+       if nlist.size > 0:
+           for n in np.nditer(nlist):
+               print("  atom {} with ID {}".format(n,tags[n]))
+
 **Methods:**
 
 * :py:meth:`lammps.get_neighlist() <lammps.lammps.get_neighlist()>`: Get neighbor list for given index