Integrate Parcels v4 into VirtualShip

[j-atkins]

Integrate Parcels v4 into VirtualShip, can be an experimental branch for now and merged/released when Parcels v4 itself is released.

This should see a lot of code evaporate, e.g. space_time_region, and ease some existing bottlenecks.

[erikvansebille]

Note that the basis of this already exists: the #220 PR