Color pressure plot according to current SAC rate

Similar to color indicating vertical speed in the profile plot we now use
color in the tank pressure plot to indicate current SAC rate.

We use a 45 sec sliding window to make sure we cover at least two breaths
for each current SAC sample to avoid artificial oscillation based on
breathing rhythm for corputers with high sample resolution.

Not sure about the color coding that I'm using right now - it's green-ish
for SAC rates under 15l/min ~= .55cuft/min and turns yellow and red as you
go higher. That seems to work well for me, but for other divers this may
be way off (or at least not as useful). Maybe this should be configurable?
This is a lot more diver specific than the vertical velocity where there
are clear recommendations based on safety considerations on what is good
and bad.

As a side effect, this removes the color coding that showed you whether
you were looking at pressure data from samples (green) vs. interpolated
pressure data (yellow). Not sure if people really want to see that. We
might be able to indicate this differently (I am thinking different line
width or transparency or something along those line)

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