Change how a Python pair style is loaded

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.

examples/python/potentials.py

@@ -0,0 +1,34 @@
+from __future__ import print_function
+
+class LAMMPSLJCutPotential(object):
+
+    def __init__(self):
+        self.pmap=dict()
+        # set coeffs: eps, sig, 48*eps*sig**12, 24*eps*sig**6,
+        #                        4*eps*sig**12,  4*eps*sig**6
+        self.coeff = {'lj'  : {'lj'  : (1.0,1.0,48.0,24.0,4.0,4.0),
+                               'NULL': (0.0,1.0, 0.0, 0.0,0.0,0.0)},
+                      'NULL': {'lj'  : (0.0,1.0, 0.0, 0.0,0.0,0.0),
+                               'NULL': (0.0,1.0, 0.0, 0.0,0.0,0.0)}}
+
+    def map_coeff(self,name,type):
+        if name in self.coeff:
+           self.pmap[type] = name
+        else:
+           raise Exception("cannot match atom type %s" % name)
+
+    def compute_force(self,rsq,itype,jtype):
+        coeff = self.coeff[self.pmap[itype]][self.pmap[jtype]]
+        r2inv  = 1.0/rsq
+        r6inv  = r2inv*r2inv*r2inv
+        lj1 = coeff[2]
+        lj2 = coeff[3]
+        return (r6inv * (lj1*r6inv - lj2))
+
+    def compute_energy(self,rsq,itype,jtype):
+        coeff = self.coeff[self.pmap[itype]][self.pmap[jtype]]
+        r2inv  = 1.0/rsq
+        r6inv  = r2inv*r2inv*r2inv
+        lj3 = coeff[4]
+        lj4 = coeff[5]
+        return (r6inv * (lj3*r6inv - lj4))