[!264] Implement ILU and ILDL preconditioners for scalar matrix entries

Merge branch 'implement-ilu-double' into 'master'

ref:core/dune-istl This merge request shifts to a new implementation strategy:
There are methods toVector and toMatrix now that interpret scalars as
FieldVector or FieldMatrix objects, respectively. That allows to get rid of
the Hybrid::ifElse pattern, which is legible but longer.

See merge request [!264]

  [!264]: gitlab.dune-project.org/core/dune-istl/merge_requests/264