ENH: sparse/dsolve: make the L and U factors of splu and spilu accessible

The L and U factors in sparse direct LU decomposition computed by SuperLU
are not currently accessible from Python. This PR makes them accessible.

The PR also renames the superlu object returned by splu() and spilu() to something
a bit saner, and links it to the documentation.

Goes on top of gh-3367 (new commits in this PR start from 1f49ccd)