Expose average_per_body in result requests

[PProfizi]

Description of the feature

The average_per_body feature was added in #690 but has not been exposed to the static result request methods

Steps for implementing the feature

No response

Useful links and references

No response

[janvonrickenbach]

@PProfizi Just FYI: I have not exposed this partly because we still have to think about a good API. Currently the API is quite low level as it allows you to select the properties on which to split the mesh. I would wait until this API is consolidated before exposing it in the (public) result functions.