Translate velocity units in planner settings

Signed-off-by: Sergey Starosek <sergey.starosek@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: don't track cylinder pressure if working pressure is 0

The pressure graph will go from 0 to negative - one could make a weak
argument that this would at least tell you how much pressure you'd need in
the cylinder to start with, but that's kinda lame.

Fixes #615

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>