add_sb_package => add_sb_python_package for Shapely
CDAT_USE_SYSTEM_XXX => SB_ENABLE_XXX check
Add external lib directory to library path
add_cdat_package => add_sb_python_package for pylibxml2
Allow Python package name to be specified

Add named parameter to add_sb_python_package(...) to allow the
Python package name to be specfied, which in some cases may be
different to the package name used in the super build.
Fix version past for numpy
add_sb_package => add_sb_python_package for pyzmq
Turn off ZMQ by default