batch-fetch parent and child nodes. depth is numeric and can't be null

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | relates to #392
| License       | MIT
| Doc PR        | none needed

the BC break is that depth now must be numeric as it is in phpcr. it made no sense to have a non-numeric depth so this should not  break for anybody.