Inverse LSR1Operator

[MaxenceGollier]

We have an InverseLBFGSOperator but not an InverseLSR1 one, we should add it.

[dpo]

Since it’s likely to be indefinite, it will be of very limited use (if any). The forward operator is useful in trust-region methods. The inverse operator would be useful in a linesearch method, but since it won’t guarantee that we find a descent direction, I’m not sure what advantage it would have.

[MaxenceGollier]

Ok, we should remove a few things in src/lsr1.jl and in the documentation as well then because I think it is confusing, I will open a PR.