ENH: add ROOTGREAT constants (symmetry with ROOTSMALL)
ENH: add some scalar constants for .org compatibility (#1881) Although it can very much be a moving target, it can prove partly useful to have some compatibility constants/methods. - The wholesale change of 'GREAT' -> 'great' etc (JAN-2018), makes user coding for multiple versions problematic. When COMPAT_OPENFOAM_ORG is defined, now define constants (aliases) named as per the openfoam.org version. Values, however, remain identical. - For type-safe dictionary value retrieval, we have the templated get<> methods added around NOV-2018 and deprecated the lookupType method. The .org version followed suit in NOV-2019, but opted for renaming the templated lookupType method as a templated 'lookup' method. Using this is discouraged, but allowed when COMPAT_OPENFOAM_ORG is defined.