Changing my mind, `interpolate_eddy` only interpolates, it does not
write the interpolated eddy, this seems clearer. So it can be a
function instead of a subroutine and we call `write_eddy` from
overlap. We need to add arguments `hshp_extremum`, `hshp_outermost`
and `hshp_max_speed` to overlap. The main program
`test_successive_overlap` now has to create shapefiles.