`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.
