Expose lapack routines for solving least squares problems

[paulkoerbitz]

Lapack has the xGELS xGELSY xGELSS and xGELSD methods for solving least squares problems (see here), by QR factorization or SVD. It would be nice to expose these to enable direct solving of least squares problems via ndarray-linalg. In fact, I wanted to do that in this PR but resolved to solving the linear equations since these methods were not available.