Allow to distinguish between Pardiso and MKLPardiso

[j-fu]

Hi,

At the moment, in the Pardiso extension:

LinearSolve.jl/ext/LinearSolvePardisoExt.jl

Line 28 in 3881230

solver = if Pardiso.PARDISO_LOADED[]

it is not possible to distinguish between Pardiso and MKLPardiso
Since MKL Pardiso was forked off in the early 2000's these are now quite different codes. I think we need to be able to distinguish between them.

[ChrisRackauckas]

Right now it uses Pardiso if Pardiso is installed and loaded, and MKLPardiso otherwise. The reason is because MKLPardiso automatically installs and Pardiso takes a whole crazy installation process, so if you went through all of those steps we assume you want to use it. But it would be very straightforward to add a boolean that allows someone which one to choose. Since it's just a high level branch we can just add mkl=nothing as a default kwarg in the PardisoJL type which chooses which one to load.

[j-fu]

Solved with
#506