Implement to_dense (MatGeneric abstract method) in all subclasses both on GPU and CPU.

src/faust_linear_operator/CPU/faust_MatButterfly.h

src/faust_linear_operator/CPU/faust_MatButterfly.hpp

src/faust_linear_operator/CPU/faust_MatDense.h

src/faust_linear_operator/CPU/faust_MatDense.hpp

src/faust_linear_operator/CPU/faust_MatDiag.h

src/faust_linear_operator/CPU/faust_MatDiag.hpp

src/faust_linear_operator/CPU/faust_MatPerm.h

src/faust_linear_operator/CPU/faust_MatPerm.hpp

src/faust_linear_operator/CPU/faust_MatSparse.h

src/faust_linear_operator/CPU/faust_MatSparse.hpp

src/faust_linear_operator/GPU2/faust_MatButterfly_gpu.h

src/faust_linear_operator/GPU2/faust_MatButterfly_gpu.hpp

src/faust_linear_operator/GPU2/faust_MatDense_gpu.cpp.in

src/faust_linear_operator/GPU2/faust_MatDense_gpu.h

src/faust_linear_operator/GPU2/faust_MatDense_gpu.hpp

src/faust_linear_operator/GPU2/faust_MatGeneric_gpu.h

src/faust_linear_operator/GPU2/faust_MatPerm_gpu.h

src/faust_linear_operator/GPU2/faust_MatPerm_gpu.hpp

src/faust_linear_operator/GPU2/faust_MatSparse_gpu.h

src/faust_linear_operator/GPU2/faust_MatSparse_gpu.hpp