Adding explicit explanation how to add tracers in FieldSet.from_nemo()

[erikvansebille]

Fixes #1700

[VeckoTheGecko]

Are there any usecases then for providing T-points to the Nemo constructor?

Do you think we should also add additional validation to the from_nemo function? Maybe if there's no usecase for T-points, we can warn if they provide >2 unique dimensions for grids.

[erikvansebille]

Do you think we should also add additional validation to the from_nemo function? Maybe if there's no usecase for T-points, we can warn if they provide >2 unique dimensions for grids.

Well, users may want to add wind or stokes drift from other models, that would then be on a different grid. So I don't think we can enforce only 1 Grid when the velocities are from_nemo