Topology:unstructured_voronoi

MPAS unstructured spherical centroidal-Voronoi (icosahedral) C-grid on the unit sphere: NCELLS Voronoi cells, NEDGES edges, NVERTICES triangle vertices, with cell valence bounded by MAXEDGES (on a closed sphere the Euler relations give NEDGES = 3*(NCELLS - 2) and NVERTICES = 2*(NCELLS - 2), but the three counts stay independent metaparameters so planar or open meshes can reuse the file).