extraction of eddies and overlapping.

Procedure dispatch_snapshot will be called by the overlap program and
will not write eddies. So remove the calls to write_eddy from
dispatch_snapshot and move them to test_get_snapshot and
test_read_snapshot.

Do not create files isolated_nodes_1.csv and number_eddies_1.csv in
test_get_snapshot and test_read_snapshot, that will be done by the
overlap program.