these objects are only used by flow_legacy, so not having to deal with
them anymore lets non-legacy flow avoid to jump through a lot of hoops
for the sake of having a common API.
this required a fork of the NonlinearSolver and AdaptiveTimeStepping
classes. this is not a problem because the original classes would get
pruned to look like the new ones once flow_legacy gets moved out of
the opm-simulators module.