fix mesh_get_centroids() for cells of lower topological dimension
fix mesh_get_centroids() for cells of lower topological dimension

lokik/sfepy,lokik/sfepy,sfepy/sfepy,rc/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,vlukes/sfepy,lokik/sfepy,BubuLK/sfepy,lokik/sfepy,rc/sfepy,sfepy/sfepy