During the first fixed point iteration, velocityIterate and the old
velocity are identical except for the new BC: velocityIterate has
already had them imposed.

To simplify the control flow, kill v completely.

Note that since we always make two fixed point iterations and this issue
only occurs during the first step, this change only affects
computational results as (a reduction of) noise

This was only a temporary workaround for problems that no longer exist.

The previous solution assumed that some sections consisted of a single
face on the coarse grid

This allows us to return a GlobalFriction and thus hide some information