Settling (fall) velocity must be corrected when vertical grid layers are moving

After a discussion during the COHERENS developer meeting (@llepers @pluyten @acapet), it seems that settling (fall) velocities in sediment and biology modules are not corrected to account for moving vertical layers.

In theory, a particle that is not moving vertically (zero settling velocity in z-coordinates) should have a non-zero settling velocity in a sigma-coordinate system, compensating for the fact that sigma layers are moving vertically. It's all relative :-)

This needs to be checked and corrected asap!