Enable the use of functions as preconditioners

[mohdibntarek]

Often the preconditioner can be defined as a function instead of a type with ldiv! method. For example, see JuliaLinearAlgebra/IterativeSolvers.jl#258. It would be nice to be able to wrap the function in a type PreconditionerFunction and pass that to iterative solvers seamlessly.

[bodhi91]

Is there a PreconditionerFunction for use? I have coded a multigrid preconditioner which returns a function accepting a vector as its argument. I aim to pass this function to the lobpcg. I was wondering if I have to write my own wrapper trying to overload ldiv! or if there is an existing PreconditionerFunction ready for use?

[JeffFessler]

Assuming a preconditioner is a linear mapping, how about using LinearMaps.jl instead of a function?