Multiple GPU support

[fangq]

To launch MCX simulations on multiple GPUs to maximize computational throughput. This can be done similar to MCXCL, by creating multiple threads and launch kernels to multiple GPUs.

[fangq]

Multiple GPU is now supported in MCX. see

60238c4

use "-G" followed by a "01" string to enable and disable multiple GPUs. For example, -G 1111 will use all 4 available GPUs; -G 0101 will use the 2nd and 4th GPU.

use "-W" followed by a comma-separated numbers to specify the relative workload for multiple GPUs. For example "-G 0101 -W 20,60" this will run 25% of the total photons on the 2nd GPU, and 75% on the 4th GPU.