ImpNodeCache is specific to Octree so it's best that it lives
near the source.

Change-Id: I35e937343312b0ab18ed1a4dcaa067ea01a0191f
Reviewed-on: https://gerrit.libreoffice.org/70736Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>