Don't create stop at time zero when re-editing manually added dive

To our addStop method time = 0 & depth = 0 are a magic flag. And since we
always start at the surface, that is assumed, anyway. So let's just not
add a stop for that when re-creating a plan from a dive.

Fixes #254

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