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

examples/python/in.pair_python_table

@@ -0,0 +1,32 @@
+# 3d Lennard-Jones melt
+
+units		lj
+atom_style	atomic
+
+lattice		fcc 0.8442
+region		box block 0 10 0 10 0 10
+create_box	1 box
+create_atoms	1 box
+mass		* 1.0
+
+velocity	all create 3.0 87287
+
+pair_style	python 2.5
+pair_coeff	* * potentials.LJCutMelt lj
+
+# generate tabulated potential from python variant
+pair_write      1 1 10000 rsq 0.01 2.5 lj_1_1.table LJ
+
+pair_style      table linear 10000
+pair_coeff      1 1 lj_1_1.table LJ
+
+neighbor	0.3 bin
+neigh_modify	every 20 delay 0 check no
+
+fix		1 all nve
+
+thermo		50
+run		250
+
+shell rm lj_1_1.table
+