Cuthill-McKee: Tweaked implementation, resulting in a reduction of execution times by 40 percent.

The performance gain was mainly obtained through replacing std::vector<> by std::pair<> in a container.
In addition, integer conversions are now removed, resulting in less conversion warnings at -Weverything at Clang.
Reordering: Improved speed of advanced Cuthill-McKee by 40%, inverted permutations

The permutation array returned by viennacl::reorder() routines now maps the old node index to the new node index,
i.e. permutation[old_index] = new_index.
This is more appropriate for use in other algorithms, where one had to frequently invert the permutation so far.