Create generic unstructured dataset, resembling FESOM2, from delaunay triangulation with sigma coordinate variable topography

[fluidnumerics-joe]

Relates to https://github.com/OceanParcels/Parcels/issues/2006

Create generic unstructured dataset, resembling FESOM2, from delaunay triangulation and uniform vertical grid spacing.

The model domain

• is regional
• has variable bottom topography (e.g. gaussian hill)
• uses uniform grid spacing in sigma coordinates

The underlying fields should be placed on [vertices,face-centers] X [ vertical faces, vertical centers] and should be defined as analytical functions to help with verification of interpolation methods.

Add a sketch of a test in the tests/v4 subdirectory that is meant to exercise the field.eval