adding Pair::single() support to python pair style and examples

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

src/PYTHON/pair_python.h

@@ -42,6 +42,7 @@ class PairPython : public Pair {
   void settings(int, char **);
   void coeff(int, char **);
   double init_one(int, int);
+  double single(int, int, int, int, double, double, double, double &);
 
  protected:
   double cut_global;