Bug fixes in `test_overlap`

In procedure `write_eddy`, no need of special treatment for
interpolated eddy: the outermost contours and max-speed contours are
defined as null.

Read name of input file on the command line in script `read_overlap.py`.

Add tests with program `test_overlap` in `short_tests.json`.

Bug fix in main unit `test_overlap`: we have to call
`init_interpolated_eddy`.  In main unit `test_overlap`, change the
initial value of `flow(2:max_delta)%number_eddies` for more clarity in
the results. Bug fix in main unit `test_overlap`: valid isolated
eddies of snasphot 2 are in `flow(max_delta + 1)`, not `flow(2)`.