[!346] Remove ISTL::InverseOperator
Merge branch 'patch/remove-alt-istlinverseoperator' into 'master'
ref:dune-fem/dune-fem> removes solver/istl.hh. the file was disabled since it is missing several included files and clashes with solver/amgistl.hh
See merge request dune-fem/dune-fem!346
(cherry picked from commit e67cdf46)
fbad5c35 remove solver/istl.hh since it is missing included files and is disabled