New TransformHelper<FPP,GPU2> class and misc update in other GPU2 classes.

Add TransformHelper<FPP,GPU2> and quick tests (yet to enhance).
Refactor TransformHelper<FPP,GPU2> and TransformHelper<FPP,Cpu> into parent class TransformHelperGen (yet to finish).
Add Transform::update(), multiply(MatDense<FPP,GPU2>), update get_product, add factor iterator and begin()/end().
Update MatDense<FPP,GPU2>/MatSparse<FPP,GPU2> : move operator overload and ctor, getType.
Update MatDense<FPP,GPU2> tests.
Fix some Vect functions.
Move hpp.in GPU2 files to cpp.in as they are only template specializations (types as variables generated by cmake).