add python gsw
It would be nice to add python gsw to the standard environments. https://github.com/TEOS-10/python-gsw/ I know this package will be deprecated in favor of this: https://github.com/TEOS-10/GSW-Python, however, not all functionality is implemented in the new package yet and it apparently does not work with python2.* . So it might be best to use the older package for now.