Improved GPU selection

[malago86]

I added a few lines to retrieve the GPUs that are Idle (by using the nvidia-smi output).

• Idle GPUs will be utilized first when running MCGPU
• If there is not enough idle GPUs, the process will be distributed on the available idle GPUs
• If there is no idle GPU available, it will wait until at least one is available

Take a look at the changes and let me know what you think! There is room for improvement, but the idea is there and seems to be working so far!