This reverts commit 4d915dba.

This is to avoid confusion to what LAMMPS considers to be an
integrator like Verlet and RESPA.

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.