Improve tank pressure plot for computers that create "gaschange" events

Improve tank pressure plot for computers that create "gaschange" events

This was exposed by the test dives, but it shows up in small ways with
real dives from some dive computers like the Suunto Vyper Air.

We now insert synthetic plot_info entries that match the gas change event;
to make this look smoother we insert either two events (one for the old
tank, one a second later for the new tank) if there is no sample at the
time of the event, or one additional event (and move the real sample back
by one second) if there is a sample at the time of the event.

This does expose another issue with some dives from Linus' computer where
the pressure in the samples dips below the end pressure noted for the tank
- which creates an odd "yellow up-tick" at the end of using the first tank
in the plot. Maybe we should not insert a synthetic "last of old tank"
event if we have a sample with valid pressure in the last NN seconds
before the gas change?

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