Improve command line arguments

[WillNilges]

I've found that the Help message is incorrect in its discription of how to lay out the command line arguments. You need to specify threads, blocks, and CPU cores.

my-laptop:CudaOtsu:% ./build/CudaOtsu ~/Pictures/sample_images/good/board.png 1024 2 12 --gpu

Setting OpenMP threads num to 12 threads
        [CPU-OpenMP] Threshold value: 168
CPU-OpenMP binarization taken 4.204969 seconds

        [GPU] Histogram calculated in 12.165824 milliseconds 

        [GPU] Histogram pixels: 14505216 

        [GPU] Threshold calculated in 0.043008 milliseconds 
        [GPU] Threshold value: 168

        [GPU] Binarized in 16.961857 milliseconds 

        [GPU] Total calculation time: 29.170689 milliseconds

If I don't include that 12, then it will run the CPU implementation.