removed a bug in fix_neb.cpp which prevented the freeend to work properly, plus added an example for the neb freeend

removed a bug in fix_neb.cpp which prevented the freeend to work properly, plus added an example for the neb freeend

- make all python potential classes derived from LAMMPSPairPotential
  which contains shared functionality. We currently don't check
  for supported atom types. may want to add that again later.
- keep track of skipped atom types in the C++ code.
- add test against units setting. must set self.units='...' in constructor
- make compute_force method consistent with Pair::single() in LAMMPS and return force/r instead of force.
- rename potentials.py to py_pot.py
- update test runs. some small tweaks.

with the single function, python pair styles can be massively
sped up and made compatible to accelerators, as one can translate
the analytic force and energy functions through LAMMPS into suitable
tables and then simply use the on-the-fly tables for production runs

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