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/example.py

@@ -0,0 +1,36 @@
+# this example requires the LAMMPS Python package (lammps.py) to be installed
+# and LAMMPS to be loadable as shared library in LD_LIBRARY_PATH
+
+import lammps
+
+def callback(caller, ntimestep, nlocal, tag, x, fext):
+    """
+    This callback receives a caller object that was setup when registering the callback
+
+    In addition to timestep and number of local atoms, the tag and x arrays are passed as
+    NumPy arrays. The fext array is a force array allocated for fix external, which
+    can be used to apply forces to all atoms. Simply update the value in the array,
+    it will be directly written into the LAMMPS C arrays
+    """
+    print("Data passed by caller (optional)", caller)
+    print("Timestep:", ntimestep)
+    print("Number of Atoms:", nlocal)
+    print("Atom Tags:", tag)
+    print("Atom Positions:", x)
+    print("Force Additions:", fext)
+    fext.fill(1.0)
+    print("Force additions after update:", fext)
+    print("="*40)
+
+L = lammps.lammps()
+L.file("in.lammps")
+
+# you can pass an arbitrary Python object to the callback every time it is called
+# this can be useful if you need more state information such as the LAMMPS ptr to
+# make additional library calls
+custom_object = ["Some data", L]
+
+L.set_fix_external_callback("2", callback, custom_object)
+L.command("run 100")
+
+