i chose to split in a separate _impl file because this code is so
generic that there may be downstream users who want to use on other
matrix types than what we use in opm-simulators.
We got compile errors like:
/home/build/opm-simulators/opm/simulators/linalg/FlexibleSolver1.cpp:24:1: required from here
/usr/include/dune/istl/ilu.hh:140:29: error: 'double' is not a class, struct, or union type
without this patch.
Hence we use the new internal ILU functions if available.
