Allow increasing CUDA thread block size

This change parametrizes the CUDA kernel to allow increasing the number
of threads per block by processing multiple j-clusters concurrently on
additional pairs of warps. The change supports 1-, 2-, and 4-way
concurrent j-cluster processing resulting in 64, 128, and 256 threads
per block, respectively.
Due to register limitations, on current CUDA architectures the version
with 64 threads/block (equivalent with the original kernels) is fastest.
The latter configurations using 128 and 256 threads are 3-4% and 10-13%
slower on CC 3.5/5.2, respectively.

Change-Id: I7c12a826f9347f724827320184628d6b310c1424