Ensure to return sparse factors in Faust::Transform object resulting of...

Ensure to return sparse factors in Faust::Transform object resulting of Palm4MSA and HierarchicalFact for the two wrappers.

For matfaust rather to return a matrix factor list, directly return a TransformHelper (because this is the wrapper object to access a Faust::Transform containing the factors). For pyfaust, it was already done this way.