This is copying the dive editing code. It uses an OO design with
virtual functions for getting and setting the values. It doesn't
use templates though, as both fields of strig type. This feels
a bit over-engineered, but it is 1) consistent with the dive edit
code and 2) the number / types of dive trip fields might increase.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
