[WIP] Swap serial lu for MPI parallel qr using MPIQR.jl

[jwscook]

This PR removes the lu serial solve and replaces it with an MPI block-parallel QR decomposition via MPIQR.jl. This PR only changes the code relevant to the actual solve, not the code for configuration. This means the rest of the code still referes to the lu solve, when in fact it will performing a qr solve.

[jwscook]

Right now MPIQR.jl is unregistered so you have to pkg> add git@github.com:jwscook/MPIQR.jl.git

[johnomotani]

In case someone else wants to try this

add git@github.com:jwscook/MPIQR.jl.git

gave me a prompt asking for a private key to access Github, but just repeated the prompt when I gave it the key. Using instead

add https://github.com/jwscook/MPIQR.jl.git

just worked.

[jwscook]

LU decomposition is able to exploit sparsity whereas QR decomposition is not. This means there's a huge performance gap between the two approaches. An example matrix has 2% non-zeros, hence a dense method is not going to win-out.

For reference, and thanks to @johnomotani:

LU solvers, running in serial:

sparse:
ldiv! 13.0ms
lu! 1.12s
lu 1.52s

dense:
ldiv! 102ms
lu 72.5s