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_melt

@@ -12,7 +12,7 @@ mass		* 1.0
 velocity	all create 3.0 87287
 
 pair_style	python 2.5
-pair_coeff	* * potentials.LJCutMelt lj
+pair_coeff	* * py_pot.LJCutMelt lj
 
 neighbor	0.3 bin
 neigh_modify	every 20 delay 0 check no
@@ -30,7 +30,7 @@ clear
 read_restart    melt.restart
 
 pair_style	python 2.5
-pair_coeff	* * potentials.LJCutMelt lj
+pair_coeff	* * py_pot.LJCutMelt lj
 
 fix		1 all nve
 
@@ -45,7 +45,7 @@ atom_style	atomic
 read_data       melt.data
 
 pair_style	python 2.5
-pair_coeff	* * potentials.LJCutMelt lj
+pair_coeff	* * py_pot.LJCutMelt lj
 
 neighbor	0.3 bin
 neigh_modify	every 20 delay 0 check no