MergingTransformer should also support non vector inputs
The current implementation of the merging transformer looks like this :
template <typename Out, typename In, typename Traits_>
struct MergingTransformer<Out( const vector_of_const_<In>& ), Traits_, true>
so we take a vector of inputs and merge it into an output.
There is a need to make it something like
template <typename Out, typename In, typename... OtherIns, typename Traits_>
struct MergingTransformer<Out( const vector_of_const_<In>&, OtherIns... ), Traits_, true>
so that other inputs, not under the form of a vector are possible.
This is in particular needed in the LHCb framework to be able to use conditions in these algorithms, as these are exactly extra inputs, marked as conditions via the Traits.