`NEW_PROP_TAG` is now a definition and not just a declaration.
Eliminate superfluous declarations, include headers with definitions.
Make one necessary forward declaration explicit.
The default was 0 for flow and 3 for flow_blackoil_dunecpr. Now we
always use 3. This is safe as it will not be used for any other
solvers than the flexible ones.
Also use it in flow_blackoil_dunecpr.cpp. Adds new command-line parameter,
--linear-solver-configuration-json-file, to read linear solver config from
JSON-format file at runtime.
