The dive_table will be converted into a table of owning pointers.
Since the trip has only non-owning pointers to dives, turn
its dive_table into an std::vector<dive *>.
Add a helper functions to add/remove items in a sorted list.
These could be used elsewhere.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>