callable(I, T) returns True when I is an object of a type T or
of a subtype of T. Also implemented a measure to avoid an
infinite loop when user-defined predicate and function classes
do not properly overload the __call__ method (including the
cases of directly instantiating the base classes such as
UnaryPredicate0D and BinaryPredicate1D).
Other methods could easily be supported in the future. The method now works as planned for the contour style. For style modules with Python shaders, there still is a problem that I will fix right away.
