This new option allows a user to select a new destination tank for an
existing "Gas Change" event. This is useful when Subsurface's heuristics
get tanks wrong after an import from a divecomputer. The use-case arose
from sidemount divers with air-integrated transmitters as well as carrying
a deco tank.
Signed-off-by: Michael Werle <micha@michaelwerle.com>