Implements a class loader which takes a fully qualified Python class
name, loads the module and creates an object instance.

To add flexibility, the current working directory and the
directory specified by the LAMMPS_POTENTIALS environment variable are
added to the module search path.

Allows to call a python function at defined points in the integration loop

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@13238 f3b2605a-c512-4ea7-a41b-209d697bcdaa