ENH: sparse/dsolve: use tp_getset instead of getattr in _superluobject.c

This is a cleaner solution.
ENH: sparse/dsolve: make the L and U factors of splu and spilu accessible from Python
ENH: sparse/dsolve: rename the superlu object, and expose and document it properly
DOC: sparse/dsolve: update versionadded
Merge branch 'pr/3375' into master.

Review at https://github.com/scipy/scipy/pull/3375