Bruhat gen

[QuentinHsr]

This PR implements the Bruhat and Compact Bruhat storages for left-triangular quasi-separable matrices and related routines:

• the computation of the quasi-separability order from a dense matrix
• the computation of a these 2 representations from dense matrix
• the product of a compact Bruhat matrix with a rectangular dense matrix

[ClementPernet]

After a thorough review, and rounds of edits, the code passes the test-suite and runs in very nice timings! Merging.