add support for multiple input arguments to Merging{,Multi}Transformer
Add support for a Merging{,Multi}Transformer with signature Out(vector_of_const_<In> const&...)
respectively std::tuple<Out...>( vector_of_const_<In> const& ... )
i.e. have an arbitrary number of input vectors instead of only allowing a single input vector.
Edited by Gerhard Raven