Clean up the handling of surface pressure

There are two ways to look at surface pressure. One is to say "what was
the surface pressure during that dive?" - in that case we now return an
average over the pressure reported by the different divecomputers (or the
standard 1013mbar if none reported any).

Or you want to do specific calculations for a specific divecomputer - in
which case we access only the pressure reported by THAT divecomputer, if
present (and fall back to the previous case, otherwise).

We still have lots of places in Subsurface that only act on the first
divecomputer. As a side effect of this change we now make this more
obvious as we in those cases pass a pointer to the first divecomputer
explicitly to the calculations.

Either way, this commit should prevent us from ever mistakenly basing our
calculations on a surface pressure of 0 (which is the initial bug in
deco.c that triggered all this).

Similar changes need to be made for other elements that we currently only
use from the first divecomputer, i.e., salinity.

Reported-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>