New "fast" routine to create simplices used only for
visualization. This speeds up the load time of large tetrahedral
(volume) meshes by roughly 50%, and brings Read_Mesh.cpp pretty close
to mshsort.cpp performance-wise. Since Read_Mesh does many more things
(all the Tree/List queries to create elementary/physical entites +
partitions), this is actually pretty good.

Loading a 1.4 million tets mesh file on my 1.5 GHz Linux machine now
takes about 17 seconds.