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/in.fix_python_integrate_melt

@@ -0,0 +1,23 @@
+# 3d Lennard-Jones melt
+
+units		lj
+atom_style	atomic
+
+lattice		fcc 0.8442
+region		box block 0 10 0 10 0 10
+create_box	1 box
+create_atoms	1 box
+mass		* 1.0
+
+velocity	all create 3.0 87287
+
+pair_style	lj/cut 2.5
+pair_coeff	1 1 1.0 1.0 2.5
+
+neighbor	0.3 bin
+neigh_modify	every 20 delay 0 check no
+
+fix		1 all python/integrate py_integrate.NVE
+
+thermo		50
+run		250