Make Python an optional dependency for LALSuite packages

- Use LALSUITE_REQUIRE_PYTHON() to require a minimum Python version
- Use LALSUITE_CHECK_PYTHON() to check for Python
- Require python >= 2.6 for SWIG Python wrappings and wrapping generation
- Python-related rules must be surrounded by HAVE_PYTHON conditionals
Use Python wrapper scripts instead of self-executing headers

- Python wrapper sets PYTHONPATH as appropriate, then executes PYTHON
- Wrapper is then used as a #! command, which avoids interfering with
  Python code (e.g. module-level documentation in """ strings)
- Wrapper $(builddir)/.python_wrapper used for uninstalled scripts; when
  installed, wrapper substituted with $(pkgdatadir)/python_wrapper
- Except if LAL package is installed into same executable prefix as Python;
  then $(PYTHON) is used directly and no wrapper is installed
LALApps: use lalsuite_python.am to generate executable Python scripts