Clarify FaustMulModeOpt Torch based methods and update their names and doc.

src/faust_linear_operator/CPU/faust_TransformHelper.hpp

src/faust_linear_operator/CPU/faust_prod_opt.h

src/faust_linear_operator/CPU/faust_torch.cpp

wrapper/matlab/+matfaust/@FaustMulMode/FaustMulMode.m

wrapper/python/pyfaust.py

wrapper/python/src/FaustCoreCppCPU.cpp.in

wrapper/python/src/FaustCoreCppGPU.cpp.in