Accept negative pressure readings

Sometimes the planner can produce negative pressures (i.e. when the
cylinders are not properly configured) or when the usser ignored
gas management (for whatever reason). When such a dive gets saved and
reread we should not display a further "Strange pressure reading"
warning on the command line.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Statistics: change our notion of "bogus SAC rate"

With a pSCR it is entirely possible to have a SAC rate below 2.8l/min.
Since we still don't want to include SAC rates of 0 let's change the
cutoff to 0.1l/min.

Fixes #624

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