Reimplement completely pyfaust.Faust.__getitem__(): now it returns a Faust...

Reimplement completely pyfaust.Faust.__getitem__(): now it returns a Faust object instead of a ndarray.

This is a way faster than doing the multiplication of factors.

- Modifying Faust::TransformHelper to represent the slices on an underlying Faust::Transform object without duplicating it (a Faust object on wrapper uses the same underlying Faust::Transform object for an original Faust and its sliced one).
Function Faust::TransformHelper::slice() request the 2D slice.

- Adding Faust::MatGeneric/Matdense/MatSparse::get_cols()/get_rows() functions.
- Adding optional argument cloning_fact to Faust::Transform(std::vector<...> factors,...) and Transform::get_fact() for optimization (factor matrices are not copied but just their pointers when cloning_fact == false).
- Modifying unit tests consistently.
- Strengthening Faust.__getitem__() furthermore in comparison to the previous version (more errors and arguments handling).
- Update CMakeLists.txt to add src/faust_linear_operator/CPU/faust_Slice.cpp

Matlab binding is left to do soon.