Remove procedure `interpolate_eddy`

Remove procedure `interpolate_eddy`. I realized that the longitude of
one of the overlapping eddies must be shifted. It seemed awkward to
pass variable m from overlap as an argument to
`interpolate_eddy`. Passing separately the useful components of
overlapping eddies made the procedure bulky, not worth creating for
three lines of computation.

As a consequence, inline the interpolation of eddies in procedure
overlap. An advantage is that part of the processing that was in
`interpolate_eddy` can migrate out of the interpolation loop.

As another consequence, remove program `test_interpolate_eddy` and
correponding test.

Upgrade procedure `candidate_overlap` to deal with non-successive
overlap. Add dummy argument delta.