Added possibility for line search methods to return a potential auxiliary value in their results

[zaccharieramzi]

Linesearch methods affected : Backtracking linesearch and Zoom linesearch.

This was was also used in order to correct the way auxiliary values are handled in the BFGS, L-BFGS and Nonlinear CG solvers where these methods are used.

I still need to add tests on the matching between the auxiliary value and the parameters returned.

[zaccharieramzi]

@mblondel I added the tests to check for the auxiliary value, all green in local. This can be reviewed.

[zaccharieramzi]

@mblondel squashed all commits as well