Add GPUModHandler within the new module faust_gpu_mod_utils.h/cpp as the...

Add GPUModHandler within the new module faust_gpu_mod_utils.h/cpp as the placeholder to gpu_mod library functions loading and access (removing prototype equivalent old code from FaustGPU, updating to gpu_mod@e4f1060e).

Other related change:
- Updating misc/test/src/C++/hierarchical2020_gpu.cpp.in

Other unrelated change:
- New constructor MatDense<FPP,GPU2>(MatDense<FPP,Cpu).

Minor fix:
- Removing cmake-generated files faust_gpu_mod_complexdouble.hpp faust_gpu_mod_double.hpp.