Distinguish internally between min pressure and end pressure

Distinguish internally between min pressure and end pressure

And don't artificially end dives on min pressure

This may be a problem for dive computers like Linus' Suunto Vyper Air
where the failure mode seems to be _high_ pressure readings (that's scary,
btw). If the transmitter fails at the end of the dive the pressure plot
ends with incorrect high pressure. But that's simply a bug with the dive
computer and not something that subsurface should hack around. Maybe we
should offer a way to edit the incorrect data points instead.

Always ending on the minimum pressure is definitely wrong as it causes
bogus plots when you do a valve shutdown during the dive (which means that
valid data gets plotted incorrectly).

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