eddies at date "j" in a new function candidate_overlap. This reduces
the number of nested constructs from 8 to 5. Incidentally, we can now
avoid defining needlessly polyline_1 if there is no candidate at date
"j". Also the function candidate_overlap should also be called by
non_successive_overlap.

Add argument periodic to procedure successive_overlap.

In main unit test_successive_overlap, pass to successive_overlap the
date subscript returned by read_snapshot instead of an arbitrary
value. Only print delta_in and delta_out if they are /= huge.