Support mixed Python use by honoring Python GIL

This enables support to both drive LAMMPS with a Python interpreter and
evaluating Python expressions inside of LAMMPS using that same interpreter.
Previously this has been avoided through an error message because the
binding code did not ensure that the necessary GIL (global interpreter lock)
structures exist (see issue #438).

All code paths which call Python C API functions must first acquire the
GIL through a call PyGILState_Ensure and release it with PyGILState_Release.