Generalize `test_successive_overlap`

Rename `test_successive_overlap` to `test_overlap`: we will also use
it for non-successive overlap. Accordingly, add an input variable
`max_delta` and change the size of flow to `max_delta` + 1. We read
from snapshots `flow(1)` and `flow(max_delta + 1)`. For
`flow(2:max_delta)`, we only need to define the component
`number_eddies`.