Start hooking in the deco calculations

I expanded the DiveHandler to include the actual time / depth of each
node on the graph - this way things will stay consistent if we need to
rescale the graph.

One thing that this makes obvious is that the whole design for the
planner so far assumes metric data. We need to make sure this works well
with feet instead of meters as well (and that it uses the information in
the units settings).

With this change we actually create a dive based on the plan input and
add the deco stops (if needed) to it - but we don't do anything with the
results of those calculations, yet.

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