dive- and trip-table functions are generated in dive.c by macros.
Move this macros to a new "core/table.h" header file.
Thus, these functions can be used for other tables (e.g. dive site)
and the trip function can be moved to a separate translation unit
(divelist.c being quite large already).
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
