Collect per tank SAC rate

This is a bit painful, but we basically walk the samples and pick the
valid tank from the events. And then we do a simple discrete integration
to figure out the mean depth per tank and duration per tank. And then we
assemble all that into per tank statistics.

Strangely the value calculated here seems slightly higher than one would
expect from the overall SAC rate. This inconsistency should be
investigated a bit further, but my guess it it's based on the assumption
that the DC provided mean depth is possibly more accurate than what we can
calculate from the profile.

Fixes #284

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Trigger redraw of the Dive Info pane on preferences change

If we change the units we need to redraw the info on this pane.

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