Considering previous dives for tissue loading

This (hopefully) gets the logic to find dives in the previous 48h for
tissue pre-loading right. In particular it handles the case of a planned
dive which has a number of -1.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>