Add a dive site table to each dive site to keep track of dives
that have been added to a dive site. Add two functions to add
dives to / remove dives from dive sites.
Since dive sites now contain a dive table, the order of includes
had to be changed: "divesite.h" now includes "dive.h" and not
vice-versa. This caused some include churn.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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