Allow to distinguish between Pardiso and MKLPardiso about linearsolve.jl

[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.