public int groupIndex() {
        return groupIndex;
    }
Returns an index of the group of this node. 
This is a 0-base continuous integer id, not necessarily the same as the group id assigned by the
application/node repo.
This index is called a "row id" in some places in Vespa for historical reasons.
config-model/src/main/java/com/yahoo/vespa/model/search/NodeSpec.java