Implement Jolien's self-executing header for executable Python scripts

- Modified ".py:" rule in lalsuite_python.am to generate, from myscript.py:
  * ./myscript, which sets PYTHONPATH to uninstalled locations
  * .pybin/myscript, which sets PYTHONPATH to installed location
- Python scripts are installed using gnuscripts/lalsuite_python_install_script,
  which installs .pybin/myscript in place of myscript