mat: add PivotedQR type

[vladimir-ch]

There should be a type in mat (tentative name PivotedQR) for computing QR factorization with column pivoting. We already have the underlying Lapack routine Dgeqp3.

See https://groups.google.com/g/gonum-dev/c/2-lpJLeynxQ/m/rm3KeTpnBgAJ for a "request" to have it in Gonum.