workaround for FVGridGeometry deprecation and clang

Merged Bernd Flemisch requested to merge fix/fvgridgeometry-deprecation-clang into master

If clang is used, false positive deprecation warnings are emitted in context of the change from FVGridGeometry to GridGeometry. As a workaround, place ignore pragmas around the corresponding declarations in propertysystem.hh. This omits all deprecation warnings that might be triggered by using GetPropType. Add a general message which warns a clang user about this issue and a switch to turn off that message.

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