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.

examples/python/in.pair_python_spce

@@ -3,11 +3,11 @@ atom_style	full
 
 read_data	data.spce
 
-pair_style	hybrid/overlay python 12.0 coul/long 12.0
+pair_style	hybrid/overlay coul/long 12.0 python 12.0
 kspace_style	pppm 1.0e-6
 
 pair_coeff	* * coul/long
-pair_coeff	* * python potentials.LJCutSPCE OW NULL
+pair_coeff	* * python py_pot.LJCutSPCE OW NULL
 
 bond_style	harmonic
 angle_style	harmonic