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/log.4May17.pair_python_table.g++.4

@@ -0,0 +1,99 @@
+LAMMPS (4 May 2017)
+  using 1 OpenMP thread(s) per MPI task
+# 3d Lennard-Jones melt
+
+units		lj
+atom_style	atomic
+
+lattice		fcc 0.8442
+Lattice spacing in x,y,z = 1.6796 1.6796 1.6796
+region		box block 0 10 0 10 0 10
+create_box	1 box
+Created orthogonal box = (0 0 0) to (16.796 16.796 16.796)
+  1 by 2 by 2 MPI processor grid
+create_atoms	1 box
+Created 4000 atoms
+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
+Neighbor list info ...
+  update every 1 steps, delay 10 steps, check yes
+  max neighbors/atom: 2000, page size: 100000
+  master list distance cutoff = 2.8
+  ghost atom cutoff = 2.8
+  binsize = 1.4, bins = 12 12 12
+  1 neighbor lists, perpetual/occasional/extra = 1 0 0
+  (1) pair python, perpetual
+      attributes: half, newton on
+      pair build: half/bin/atomonly/newton
+      stencil: half/bin/3d/newton
+      bin: standard
+
+pair_style      table linear 10000
+pair_coeff      1 1 lj_1_1.table LJ
+WARNING: 1 of 10000 force values in table are inconsistent with -dE/dr.
+  Should only be flagged at inflection points (../pair_table.cpp:476)
+
+neighbor	0.3 bin
+neigh_modify	every 20 delay 0 check no
+
+fix		1 all nve
+
+thermo		50
+run		250
+Neighbor list info ...
+  update every 20 steps, delay 0 steps, check no
+  max neighbors/atom: 2000, page size: 100000
+  master list distance cutoff = 2.8
+  ghost atom cutoff = 2.8
+  binsize = 1.4, bins = 12 12 12
+  1 neighbor lists, perpetual/occasional/extra = 1 0 0
+  (1) pair table, perpetual
+      attributes: half, newton on
+      pair build: half/bin/atomonly/newton
+      stencil: half/bin/3d/newton
+      bin: standard
+Per MPI rank memory allocation (min/avg/max) = 2.69 | 2.69 | 2.69 Mbytes
+Step Temp E_pair E_mol TotEng Press 
+       0            3   -6.7733675            0   -2.2744925   -3.7033435 
+      50    1.6754092    -4.794723            0   -2.2822376    5.6616601 
+     100    1.6503295   -4.7559815            0   -2.2811061    5.8051261 
+     150    1.6596603   -4.7699379            0   -2.2810699    5.7830168 
+     200    1.6371948   -4.7365549            0   -2.2813766    5.9245585 
+     250    1.6321199   -4.7288017            0   -2.2812339    5.9776124 
+Loop time of 0.313548 on 4 procs for 250 steps with 4000 atoms
+
+Performance: 344444.576 tau/day, 797.325 timesteps/s
+99.1% CPU use with 4 MPI tasks x 1 OpenMP threads
+
+MPI task timing breakdown:
+Section |  min time  |  avg time  |  max time  |%varavg| %total
+---------------------------------------------------------------
+Pair    | 0.24963    | 0.25443    | 0.2632     |   1.1 | 81.15
+Neigh   | 0.023249   | 0.023735   | 0.024497   |   0.3 |  7.57
+Comm    | 0.020689   | 0.030402   | 0.035249   |   3.4 |  9.70
+Output  | 0.00020766 | 0.00021476 | 0.00023031 |   0.0 |  0.07
+Modify  | 0.0034959  | 0.0035564  | 0.0036762  |   0.1 |  1.13
+Other   |            | 0.001206   |            |       |  0.38
+
+Nlocal:    1000 ave 1010 max 982 min
+Histogram: 1 0 0 0 0 0 1 0 0 2
+Nghost:    2703.75 ave 2713 max 2689 min
+Histogram: 1 0 0 0 0 0 0 2 0 1
+Neighs:    37915.5 ave 39231 max 36202 min
+Histogram: 1 0 0 0 0 1 1 0 0 1
+
+Total # of neighbors = 151662
+Ave neighs/atom = 37.9155
+Neighbor list builds = 12
+Dangerous builds not checked
+
+shell rm lj_1_1.table
+
+Total wall time: 0:00:00