mitk::SlicedGeomerty3D provides a setter to set the reference geometry, but it does not provide a getter. The setter sets the same geometry as reference geometry to every internal plane geometry, and mitk::PlaneGeometry provides a public setter and getter.
Since you can access the reference geometry of the sliced geometry through its composing plane geometries, there is no reason not to have a getter in mitk::SlicedGeometry3D itself.
Same applies to HasReferenceGeometry().
I will send a PR that fixes these inconsistencies in the API and ensures constness of the reference geometry.