add getProperties() to the interface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a

this would require updating PhpcrOdmTree in the sonata phpcr admin bundle with a not "implemented" exception