Implement GPU Butterfly Faust optimization in the C++ core.

src/faust_linear_operator/CPU/faust_MatDense.h

src/faust_linear_operator/CPU/faust_MatSparse.h

...aust_linear_operator/CPU/faust_TransformHelperButterfly.h

...st_linear_operator/CPU/faust_TransformHelperButterfly.hpp

...linear_operator/GPU2/faust_TransformHelperButterfly_gpu.h

...near_operator/GPU2/faust_TransformHelperButterfly_gpu.hpp