Add a variadic template ctor for TransformHelper, add push_back_() related functions and a test.

It eases the creation of TransformHelper objects by passing any arguments of type Transform, TransformHelper or std::vector<MatGeneric*> which are used to form a sequence of factors in the new TransformHelper object (in the same order and without any copy).

Modify Transform::push_back() to allow not copying a MatGeneric.

Example: see misc/test/src/C++/unit/test_TransformHelper_variadic_template_ctor.cpp.in

Other minor changes.