Extend lib interface to set fix external callback

This allows creating a callback in Python and attaching it to
a fix external instance.

examples/COUPLE/python/in.fix_external

@@ -0,0 +1,23 @@
+# LAMMPS input for coupling LAMMPS with Python via fix external
+
+units		metal
+dimension	3
+atom_style	atomic
+atom_modify	sort 0 0.0
+
+lattice		diamond 5.43
+region		box block 0 1 0 1 0 1
+create_box	1 box
+create_atoms	1 box
+mass		1 28.08
+
+velocity	all create 300.0 87293 loop geom
+
+fix		1 all nve
+fix		2 all external pf/callback 1 1
+
+#dump		2 all image 25 image.*.jpg type type &
+#		axes yes 0.8 0.02 view 60 -30
+#dump_modify	2 pad 3
+
+thermo		1