Create example Kernel using gsw package for equation of state calculation

[erikvansebille]

Now that v4 supports calling python-functions and libraries in the Kernel, one good use case would be to use the gsw package from https://teos-10.github.io/GSW-Python/ or https://github.com/jarethholt/polyTEOS10 for calculations involving the Equation of State of seawater (e.g. density from temperature, salinity and pressure), and do this directly in a Kernel.

This would then mean that we can remove the kernels in TEOSseawaterdensity.py and EOSseawaterproperties.py

[nilodna]

After #2248, is this issue still necessary?

[erikvansebille]

No, it's indeed done in #2248; I forgot to close this issue after the merge