Add --depth argument to cli import

See https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7556

Rather than setting `JAVA_OPTS=-Domero.import.depth=10`, users should be able to easily find and set a `--depth` argument on `bin/omero import`.

To test:
 * create a directory with multiple levels: `mkdir /tmp/a/b/c`
 * touch a single fake in that directory: `touch /tmp/a/b/c/deep.fake`
 * `bin/omero import --depth=1 -f /tmp/a` should show no files
 * `bin/omero import --depth=3 -f /tmp/a` should show the fake