warm-up of a multi-GPU rig.
Idea of evolution :
add a parameter -warmup_time to let user specify a warmup time between the start of each GPU startup.

-warmup_time = 30 
-> 30 s between each GPU start

-warmup_time = 0,20,30,40,50,60
-> GPU0 start initially, 1 @ 20s from nanominer startup, 2 @ 30s from nanominer startup

-warmup_time = 0,+20,+20,+20,+20,+20
GPU0 starts instantaneously, and GPUn starts 20s after GPUn-1