The undo system sets updates individual dive fields on
redo respectively undo. Make salinity such a field, since
it is changed on replanning a dive.
To do this, break out the "update salinity" functionality
into its own function, add an entry to the DiveField enum
and add the corresponding switch-case.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
6a6b992a77