Rename fix python to fix python/invoke

examples/python/in.fix_python_invoke

@@ -0,0 +1,50 @@
+# 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 1.0
+
+velocity	all create 3.0 87287
+
+pair_style	lj/cut 2.5
+pair_coeff	1 1 1.0 1.0 2.5
+
+neighbor	0.3 bin
+neigh_modify	every 20 delay 0 check no
+
+python         end_of_step_callback here """
+from __future__ import print_function
+from lammps import lammps
+
+def end_of_step_callback(lmp):
+  L = lammps(ptr=lmp)
+  t = L.extract_global("ntimestep", 0)
+  print("### END OF STEP ###", t)
+
+def post_force_callback(lmp, v):
+  L = lammps(ptr=lmp)
+  t = L.extract_global("ntimestep", 0)
+  print("### POST_FORCE ###", t)
+"""
+
+fix		1 all nve
+fix     2 all python/invoke 50 end_of_step end_of_step_callback
+fix     3 all python/invoke 50 post_force post_force_callback
+
+#dump		id all atom 50 dump.melt
+
+#dump		2 all image 25 image.*.jpg type type &
+#		axes yes 0.8 0.02 view 60 -30
+#dump_modify	2 pad 3
+
+#dump		3 all movie 25 movie.mpg type type &
+#		axes yes 0.8 0.02 view 60 -30
+#dump_modify	3 pad 3
+
+thermo		50
+run		250