I've added enum constants for how well an arg matches the arg type
required by a particular method signature.  The enums are
EXACT_MATCH, GOOD_MATCH, NEEDS_CONVERSION, and INCOMPATIBLE.
For vtkObjectBase args, the code counts the number of generations
between the required arg class and the provided arg class.  A
difference of one generation is GOOD_MATCH, two generations is
GOOD_MATCH+1, etc. getting progressively worse but never as bad
as NEEDS_CONVERSION. Lower values signify better matches.