The approach is inspired by Geiger's system-amg but we use dune-istl
aggregation AMG for it. On the fine level all unknowns attached to a cell
form a matrix block and are treated fully coupled. To form the first
coarse level system we use only the pressure component to guide the aggregation
and neglect all other unknowns on the fine level. All other level are formed
in the usual way by scalar aggregation.
Currently,it has to be requested for flow_ebos manually by passing
"linear_solver_use_amg=true amg_blackoil_system=true" to it.