Added two versions of channel convolution and noise generation:
 - accelerated via threadpool
 - accelerated using CUDA

The main difference between this and previous versions of channel
convolution implementations is that these functions take a real-world
approach to input/output where both could be split unevenly over a set of
buffers, e.g. ring buffers used in vrtsim.

The CUDA-accelerated version only works on systems with unified memory, e.g. NVidia
DGX Spark or GH