Add DILU preconditioner
This MR adds the Diagonal-ILU (DILU) preconditioner to DUNE-ISTL.
The code has been developed to add the preconditioner to OPM FLOW, see https://github.com/OPM/opm-simulators/pull/4899 .
DILU is a simplified version of the ILU(0) where only the diagonal terms are factorised, which means that we only need to store the diagonal for applying the preconditioner. More details on the method can be found here: https://netlib.org/linalg/html_templates/node64.html#SECTION00842200000000000000 .