Change GPU workload

[mastercracker]

Sounds more like a single-user problem

Not that it's something we cannot work without but there are times where I could use it. Sometimes when you are in the middle of a run and find your GPUs are overheating too much or if you want to temporarily lower your fan noises without stopping everything, adjusting the -n during a run would be useful. If I remember well, one or a couple of older versions would do that when pressing the + and - keys.