Exclude surface intervals from sac rate calculation

Exclude surface intervals from sac rate calculation

We assume every sample with a depth of less than 10cm to be on the
surface.

This does not impact our interpolated pressures (one could assume that the
diver is not breathing from the regulator when on the surface - but
without air integration that's just an assumption).

It also doesn't change our tank pressure coloring by sac rate as that
always uses the momentary sac rate. Technically speaking this might impact
the actual colors printed (as those are relative to the total sac on the
dive which may go up due to this change).

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