Next, the external nodes, edges and faces are identified.
A face is external if it belongs to only one element. If face is is external iexternfa(i) is set to 1, else it is 0.
All nodes in an external face are external. For these nodes iexternnode is set to 1, else it is 0.
All edges in an external face are external. The value for these edges in iexternedg is at first set to 1, else it is 0.