a bunch refactoring changes in the python pair style and the examples

- 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.

src/PYTHON/pair_python.h

@@ -47,6 +47,7 @@ class PairPython : public Pair {
  protected:
   double cut_global;
   void * py_potential;
+  int  * skip_types;
 
   virtual void allocate();
 };