Circumvent gfortran bug in `CMakeLists.txt`.

Prepare for overlap of eddies at non-successive dates:

- add argument `delta_out` to procedure `candidate_overlap` (not yet
used);
- add procedure `interpolate_eddy` (empty for now);
- rename `successive_overlap` to overlap because the procedure will handle both
cases, successive dates and non-successive dates;
- generalize procedure overlap to handle delta between dates.