mat: add example for polynomial regression with QR factorization
mat: add example for polynomial regression with QR factorization

Fixes #1759.

Signed-off-by: Sebastien Binet <be0c0d1298e63814a95fcb7523d1ca1345b4ca1f@cern.ch>
gonum/gonum,gonum/gonum,gonum/gonum