[Solvers] Add linear backend that uses UMFPack.
[Solvers] Include umfpack.hh required for UMFPack backend
[Documentation] Always add SuperLU backend to Doxygen
Merge branch 'feature/add-umfpack-backend' into releases/2.0

* feature/add-umfpack-backend:
  [Solvers] Include umfpack.hh required for UMFPack backend
  [Solvers] Add linear backend that uses UMFPack.
Merge branch 'releases/2.0'

Forward-port UMFPack support

* releases/2.0:
  [Documentation] Always add SuperLU backend to Doxygen
  [Solvers] Include umfpack.hh required for UMFPack backend
  [Solvers] Add linear backend that uses UMFPack.