Only use a default cylinder if it is explicitly set

We used to fall back to an AL80 default cylinder, but that meant that a
user who doesn't want a default cylinder at all had no way to indicate
that.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Only use default cylinder for first one

When downloading from a divecomputer it makes little sense to assume that
all tanks are the default cylinder.

There's a good case to be made for having a default first cylinder (you
always dive with your own cylinder, or you are always on a dive boat with
AL80 tanks), but in multi-cylinder situations this is much more likely to
cause unintended harm; for example for those dive computers that always
report their maximum number of cylinders, even if some of them aren't
used.  Here setting a default cylinder turns those entries from obviously
empty into something that appears to have meaning (i.e., cylinder type is
filled in) even though this was just a default added by Subsurface.

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