small schema /extension of existing schema for identifying connections

[lzehl]

Parcellation entities or parcellation entity versions could also represent white matter structures (fibre bundles). Currently what regional parcellation entities or parcellation entity versions they connect is not existing. In addition it is not possible to associate connectivity features to atlas PEs and PEVs. It would be great to have a small schema for this.

@tgbugs and @aeidi89 we should discuss this soonish.