Switch to Krylov.jl 0.7.3

[frapac]

Krylov.jl adds a new feature that is of direct interest for us: inplace BICGSTAB.

• Indeed, the BICGSTAB algorithm is using some intermediate arrays. When we were solving the powerflow equations, we had to allocate these auxiliary arrays each time we were calling Krylov.bicgstab.
• This PR releases this burden, as we are using the new API of Krylov.jl to instantiate the intermediate arrays once and for all, at the beginning. Then, everything runs like a bliss

But ... we could not merge this PR as is. We need to discuss the API to instantiate a new AbstractLinearSolver instance inside ExaPF. Currently, we are calling

lin_solver = LinearSolvers.KrylovBICGSTAB(J, precond)

with J power flow Jacobian, and precond a preconditioner.
I would suggest we use instead:

lin_solver = LinearSolvers.KrylovBICGSTAB(J; precond=:block_jacobi, npartitions=100)

and then instantiate the preconditioner directly inside the constructor of KrylovBICGSTAB (and other linear solvers).
What do you think?

[dmaldona]

Great PR!