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_coulomb

@@ -0,0 +1,45 @@
+units		real	
+atom_style	full
+
+read_data	data.spce
+
+pair_style	hybrid/overlay python 12.0 coul/long 12.0
+kspace_style	pppm 1.0e-6
+
+pair_coeff	* * coul/long
+pair_coeff	* * python potentials.LJCutSPCE OW NULL
+
+pair_modify     table 0
+
+bond_style	harmonic
+angle_style	harmonic
+dihedral_style	none
+improper_style	none
+
+bond_coeff	1 1000.00 1.000
+angle_coeff	1 100.0 109.47
+
+special_bonds   lj/coul 0.0 0.0 1.0
+
+neighbor        2.0 bin
+
+fix		1 all shake 0.0001 20 0 b 1 a 1
+fix		2 all nvt temp 300.0 300.0 100.0
+
+# create combined lj/coul table for all atom types
+# generate tabulated potential from python variant
+pair_write      1 1 2000 rsq 0.1 12 spce.table OW-OW -0.8472 -0.8472
+pair_write      1 2 2000 rsq 0.1 12 spce.table OW-HW -0.8472  0.4236
+pair_write      2 2 2000 rsq 0.1 12 spce.table HW-HW  0.4236  0.4236
+
+# switch to tabulated potential
+pair_style      table linear 2000 pppm
+pair_coeff      1 1 spce.table OW-OW
+pair_coeff      1 2 spce.table OW-HW
+pair_coeff      2 2 spce.table HW-HW
+
+thermo 10
+run 100
+
+shell rm spce.table
+