Revert "Revert "[la.solver.istl] add "superlu"""
Revert "Revert "[la.solver.istl] add "superlu"""

This reverts commit 420a21430c8da95582d776a40923a16d92b15a7d.

Conflicts:
	dune/stuff/la/solver/istl.hh

renemilk/DUNE-Stuff,renemilk/DUNE-Stuff,renemilk/DUNE-Stuff