In procedure `dispatch_snapshot`, rename dummy argument m to rank,
clearer.

Rename program `test_get_snapshot` to `test_get_dispatch_snap` and
make it call `dispatch_snapshot` instead of `send_snapshot`. The only
difference in result is that the program now writes files
`isolated_nodes.txt` and `number_eddies.csv`.

Check in program `test_overlap` that it is run on a single MPI process.