Add `mul!` for matrices

[gdalle]

Goal: to fix #322 by adding mul!(res::AbstractMatrix, op::LinearOperator, m::AbstractMatrix)

In practice, there are lots of things to overload and I don't know if you want to do all of them. I also don't know how you want to test this new functionality. But anyway, here's a first draft (without tests) to get the discussion started

[amontoison]

I think we should only overload mul!, transpose and adjoint for now. We support the other ones later.

[gdalle]

Okay, how about the 5-argument mul!? From what I can tell, it uses pre-allocated storage that is specific to operator-vector multiplication?

[github-actions[bot]]

Package name	latest	stable
CaNNOLeS.jl
DCISolver.jl
FletcherPenaltySolver.jl
JSOSolvers.jl
Krylov.jl
NLPModels.jl
NLPModelsModifiers.jl
PROPACK.jl
Percival.jl
QuadraticModels.jl
SolverTools.jl

[amontoison]

We can just support the 3-args mul! for now with operator-matrix products. It's only what we need for Krylov.jl.

[gdalle]

I added the minimal amount of changes and the minimal tests, can you approve the workflows?

[gdalle]

@amontoison at your disposal to add more code, tests or docs depending on what you see fit in the review

[amontoison]

Is it working with block_gmres in Krylov.jl?

[gdalle]

Yes!

julia> using Krylov, LinearOperators

julia> A, B = rand(2, 2), rand(2, 2);

julia> block_gmres(A, B)
([0.711998925881128 -0.31790146670357355; 1.0257506949284791 3.3662342660807827], SimpleStats
 niter: 1
 solved: true
 inconsistent: false
 residuals: []
 Aresiduals: []
 κ₂(A): []
 timer: 189.24μs
 status: solution good enough given atol and rtol
)

julia> block_gmres(LinearOperator(A), B)
([0.711998925881128 -0.31790146670357355; 1.0257506949284791 3.3662342660807827], SimpleStats
 niter: 1
 solved: true
 inconsistent: false
 residuals: []
 Aresiduals: []
 κ₂(A): []
 timer: 521.58ms
 status: solution good enough given atol and rtol
)