Conversions between image representations are now managed at one central place: The ImageRepresentationConverter singleton uses the proven and established registration through static template instantiation idiom to register conversion functors during static initialization. Therefore, the ConversionFunctionRegistrar registers a conversion functor to a target representation type.

As proof-of-concept implementation, the former conversion API through T::tryConvertFrom, where T is a specific image representation, has been converted to the new API and merged into imagerepresentationconversioncore.h providing a conversion functor for each campvis-core representation.

Furthermore, implemented conversion from ImageRepresentationGL to GenericImageRepresentationLocal<>.

refs #553
refs #474