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.
- If the entry could be directly inserted: a pointer to the inserted entry.
- If a dictionary merge was required: a pointer to the dictionary that
received the entry.
- Return nullptr on any type of insertion failure.
This change is code compatible with existing code since it only alters
a bool return value to be a pointer return value.
