Handle case of 0 (-1 internal) node value index meaning use a zero DOF in element mappings. Legacy of EX format.  https://tracker.physiomeproject.org/show_bug.cgi?id=3490
Handle case of 0 (-1 internal) node value index meaning use a zero DOF in element mappings. Legacy of EX format. 
https://tracker.physiomeproject.org/show_bug.cgi?id=3490

hsorby/zinc,OpenCMISS/zinc,OpenCMISS/zinc,hsorby/zinc,OpenCMISS/zinc,OpenCMISS/zinc,hsorby/zinc,hsorby/zinc