Implement H (or RRR) representation for quasiseparable matrices

[ClementPernet]

Although asymptotically more expensive (additional log n in space and time), it is expected to perform faster on small enough matrices. We need to experiment to determine whether it is worth implementing a switch between SSS and RRR.

[Tomiki3]

Branch https://github.com/linbox-team/fflas-ffpack/tree/repRRR, still under development.
Implement this.