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.