GELSY and other rank-deficient LLS solvers are not implemented

[TiborGY]

Currently PLASMA only implements GELS for solving linear least squares problems.
Unfortunately I sometimes also need to solve LLS problems where the A matrix is rank-deficient.
It would be nice if PLASMA implemented GELSY or some other LLS solver that can handle numerically rank-deficient LLS problems.

[luszczek]

Yes, it's correct: some variant of QR with column pivoting would be a nice feature. The modern options are CQRPT, PAQR, and CA-QRCP