Add prox_spcol and prox_splin pure GPU implementations along with a test/benchmark.

gpu_mod

misc/test/CMakeLists.txt

misc/test/src/C++/test_prox_splin_spcol_gpu.cpp.in

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_prox_gpu.h

src/faust_linear_operator/GPU2/faust_prox_gpu.hpp