Add/ct iter check

This merge request adds error checks in the contour tree (and merge tree) compute to raise an exception in case the algorithm can not process all edges/nodes. This can happen in case that the user provides a bad input mesh that describes, e.g., a discontinues domain, which would result in a forest of trees rather than a single tree. This change is to prevent the algorithm from getting stuck in an infinite loop in this case and rather raise an error when this case is detected.