`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.
Refactors away flow_tag.hpp by having flow_blackoil_dunecpr.cpp,
flow_onephase.cpp, and flow_onephase_energy.cpp use the updated Main.hpp
from a previous pull request. This will eliminate the previous code
duplication in Main.hpp and flow_tag.hpp discussed in PR #2521.
The previous was "file" which resulted in runtime errors
like
Program threw an exception: No linear-solver-configuration-json-file given for linear-solver-configuration=file
The flow_tag.hpp file is used for simple construction of simulators for
particular models or other compile-time choices. This updates it to
get identical behaviour as that of mainline (multi-model) Flow.
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.
