Implement MatDense<GPU2,FPP>::multiply(MatDense<GPU2,FPP>).

src/faust_linear_operator/CPU/faust_gpu_mod.h

src/faust_linear_operator/CPU/faust_gpu_mod.hpp

...faust_linear_operator/CPU/faust_gpu_mod_complexdouble.hpp

src/faust_linear_operator/CPU/faust_gpu_mod_double.hpp

src/faust_linear_operator/CPU/faust_gpu_mod_gen.hpp.in

src/faust_linear_operator/GPU2/faust_MatDense_gpu.h

src/faust_linear_operator/GPU2/faust_MatDense_gpu.hpp.in