Mohamed Tarek

This works but the allocating version errors, for a trivially fixable reason. ```julia using LinearAlgebra, IterativeSolvers cg!(rand(2,2), Diagonal(rand(2)), rand(2,2)) ``` In https://github.com/JuliaMath/IterativeSolvers.jl/blob/master/src/common.jl#L20, `size(A, 2)` should be `size(b)`.

Or actually the size is completely unnecessary for `cg`. Not sure if there is another solver that uses this function differently.

Goes part of the way: #238

Relevant: #238

I believe this is ready for a first review.

I don't mind getting rid of `tol`. Not big on the underscore either.

See this https://github.com/JuliaMath/IterativeSolvers.jl/blob/01af27fe76069e6be566abfd862664f928997638/test/cg.jl#L12 for reference. There is only one function to be defined `ldiv!(y, P, x)`.

Check out [Preconditioners.jl](https://github.com/mohamed82008/Preconditioners.jl), which lets you do `Pl = DiagonalPreconditioner([πe;πte])`.

Cool, will make a PR then.

I don't know Literate enough to review this. But it seems CI isn't giving a preview of the docs.