Fix qr mode

This PR addresses gh-2961 by adding a 'big' mode to the qr factorization modes so that true full factorizations can be obtained. The current 'full' mode results in what is commonly called a thin factorization. Because changing the meaning of 'full' and adding a 'thin' mode would be a nasty process, adding a new 'big' mode looked like the easiest way to go. If anyone has a better suggestion for the name of the new mode I'd be happy to hear it.