Add a simple table-based cns calculations

For dives where divecomputers haven't provided us with a cns, we
calculate our cns accumulated during that dive based on a simple table.

We also check if we did a dive in the prior 12 ours and grab the cns
from it and calculate how much of that still affects us.

[Dirk Hohndel: a couple of small changes: remove unnecessary check of cns
               values in the samples of the first dive computer, changed
	       the way we determine the 'previous dive' and used the end
	       time of that previous dive for the decay calculation]

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>