* Finds nodes not used in any cell and returns an array giving a new id to every node
* by excluding the unused nodes, for which the array holds -1. The result array is
* a mapping in "Old to New" mode.
* Finds nodes not used in any cell and returns an array giving a new id to every node
* by excluding the unused nodes, for which the array holds -1. The result array is
* a mapping in "Old to New" mode.