Access run-length encoded zig-zag matrices

These wrappers enable you to write code that accesses the run-length
encoded matrices in UnstructuredGrid and the like without having to
worry about the indirect accessing of the position matrix.
Make objects that contain alias for cube sizes

It is easier to address the top face with UP, rather than 5.
Provide way to do old-fashioned iteration through matrix

Getting a pair of indices that can be used in loops are probably more
readable than trying to iterate through two iterators simultaneously.
Calculate height of each element in the columns