Add prox_id for CPU, GPU in default and MatGeneric versions.

src/algorithm/constraint/faust_ConstraintGeneric.h

src/algorithm/constraint/faust_ConstraintGeneric.hpp

src/algorithm/constraint/faust_ConstraintMat.hpp

src/algorithm/constraint/faust_ConstraintType.h

src/algorithm/constraint/faust_prox_gen.h

src/algorithm/constraint/faust_prox_gen.hpp

src/faust_linear_operator/CPU/faust_prox.h

src/faust_linear_operator/CPU/faust_prox.hpp

src/faust_linear_operator/GPU2/faust_prox_gpu.h

src/faust_linear_operator/GPU2/faust_prox_gpu.hpp