Completed first version of fix python/integrate

This includes an example of how to implement fix NVE in Python.

The library interface was extended to provide direct access to atom data using
numpy arrays. No data copies are made and numpy operations directly manipulate
memory of the native code.

To keep this numpy dependency optional, all functions are wrapped into the
lammps.numpy sub-object which is only loaded when accessed.

examples/python/README.fix_python_integrate

@@ -0,0 +1,15 @@
+This folder contains several LAMMPS input scripts and a python module
+file py_integrate.py to demonstrate the use of the fix style python/integrate.
+
+in.fix_python_integrate_melt:
+This is a version of the melt example which replaces the default NVE integrator
+with a Python implementation. Fix python/integrate is used to create an
+instance of the py_integrate.NVE class which implements the required interface.
+It demonstrates how to access LAMMPS data as numpy arrays. This gives direct
+access to memory owned by the C++ code, allows easy manipulation through numpy
+operations and avoids unnecessary copies.
+
+in.fix_python_integrate_melt_opt:
+This version of melt example uses NVE_Opt instead of NVE. While this Python
+implementation is still much slower than the native version, it shows that
+simple code transformations can lead to speedups.