Shading package: Move "function" methods to base class and more refactoring

I'm planning to move the "function" methods to the PDShadingResources class. Reasons: 1) duplicate code (partly introduced by myself), 2) this will allow me to handle functions in type 4 & 5 shading. Currently I can't, because these shading context classes use a common class (GouraudShadingContext), but the PDShadingType 4 and 5 have their function methods.