Remove redundant linear sample tank pressure data

Remove redundant linear sample tank pressure data

I've seen at least DivingLog do this. If you manually enter beginning and
end pressure for a tank it will either linearize the samples in between or
offer to simulate a dive (with constant SAC rate). At least the first case
is reasonably easy to detect. We throw out those samples and ensure that
we still have valid beginning and end pressure for that dive.

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