Row reduction row swapping

[dsharlet]

Thanks for the PR. I think I tried this at one point and found it made a negligible difference, and it only works if Ab is double[][] and not double[,] (which I was experimenting with). But it's a nice cleanup for the code as it exists now.

[Federerer]

Jagged arrays generally performs faster in .net than multidimensional ones (setting an element in multidimensional array involves a method call under the hood) and as a bonus can easily swap rows in matrices ;)