WIP: organization unit tree

[floriangantner]

Is your feature request related to a problem? Please describe.
We would like to display the hierarchical strcuture of the orgunits in our CRIS in some tree.

Describe the solution you'd like
We would like to display some kind of tree and additional metrics for each node. It should be displayed on any explore section page.

Describe alternatives or workarounds you've considered
we know there is some similar implementation planned.

Additional context
We provide our implementation to support the community. It does not include tests (yet).