Multiple Nvidia GPU support added for GPU load and temperature

[lxidea]

Hi there all, this is my first time to submit an issue wishing for some contribution and suggestions.

I have two Nvidia GPUs, I wish to see the load for each of them. so I have made modifications to the file sensor.py to get the plugin supporting enumerating different GPU card by the index, such like {nvgpu0} or {nvgputemp0} that will show the info of the first card in your system.
If this feature is good enough, then maybe I can make a pull request and merge into the master.
Please have a try if you have interest at the link below

https://github.com/lxidea/indicator-sysmonitor/tree/lxidea-multi-nvgpu