Add SuperLU_DIST LinearProblem class

[jhale]

This proposes a very simple SuperLU_DIST LinearProblem class which has a similar interface to the PETSc one.

Points for discussion:

• Do we want to expose the SuperLU_DIST solvers underlying options? Symmetry might be interesting, although @chrisrichardson couldn't find any performance differences when he tried it.
• Currently there is no way to reuse the decomposition of A for repeated solves - that said, the same applies to our PETSc LinearProblem. This is possible through the low-level class, so 'advanced' users can just use that.