Cleaup: Use CXX_FLAGS not CPP_FLAGS in lib Makefiles
Issue#161: Rename gpi_embed_python to gpi_load_extra_libs since this is what it now does
Cleanup: Remove duplication of flags from header files
Cleanup: Remove some old functions and commented out code
Issue#161: Remove native_check, been replaced with native_check_create methods
Issue#161: No need for free_handle any more, a reference is always held in python for signals and callbacks are static or automatically freed in the GPI layer
Issue#161: Do not register the same GPI implementation layer twice