Add gsw package
This adds the old pure-python gsw for Python<3.5 and the new gsw wrapping the C lib for Python>=3.5.
On Wednesday, 22 October https://git.geomar.de will experience service interruptions. The login of the service will change from the old portal.geomar.de account to the normal GEOMAR login. Your GitLab user will stay the same, just the login is exchanged. Expect that you are asked to validate your mail address during your first login.
This adds the old pure-python gsw for Python<3.5 and the new gsw wrapping the C lib for Python>=3.5.