Bugfix: generalize pp graphs to allow for multi over-threshold periods

Especially in O2 decompression parts of a dive, the pp02 is typically very
close to the threshold value (normally 1.60 bar). The old implementation
of the pp profile graphs assumes that there is exacty 1 consecutive set of
samples that needs to be in the "warning color". This results in an
erroneous display of the mentioned graphs, connecting multiple episodes of
too high pp with bogus lines in between.

This fix generalizes the pp graph logic to allow for multiple segments of
high pp, each to been drawn seperately in the "warning color".

Signed-off-by: Jan Mulder <jlmulder@planet.nl>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>