Remove column iterator

This will be replaced with a better suited abstraction later.
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.
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.
Ease creation of views with helper routines

These routines can be seen as extensions to the UnstructuredGrid
structure.
Be const-correct

This allow the compiler to generate better code.
Merge pull request #17 from rolk/017_runlen

Provide abstraction to iterate over run-length encoded matrices