Using private grid._depth for negating depth

[erikvansebille]

As Parcels v3.1.0 has moved from grid.depth to grid._depth, the code for VirtualShip is now broken.

This PR fixes that, although a more proper implementation would be to add a grid.negate_depth() method to Parcels itself?

[erikvansebille]

The other issue that causes the CI to fail is not easily fixed within virtualship; it has to do with a bug in a new warning in Parcels, see OceanParcels/Parcels#1747.

In any case, because it's 'only a warning', it doesn't affect the functionality of virtualship. We probably have to wait until Parcels is patched, @VeckoTheGecko?

[VeckoTheGecko]

@erikvansebille I wonder if a better solution would be to pin parcels<3.1.0 for the timebeing? That way we can implement .negate_depth and fix the warnings in parcels without having to push out a new release, and virtualship can be good to go for students.

[erikvansebille]

Yes I agree; pinning to <3.1.0 (or even 'not 3.1.0'; is that possible?) might be a better solution. Can you implement?

[ammedd]

Just so you know. There is no rush to fix things for virtual ship. I plan to have the next group of students working with the code early February.