Add GPU support

[hugy718]

This PR allows task executor to access and schedule NVIDIA GPUs, Docker has support for accessing NVIDIA GPUs https://docs.docker.com/config/containers/resource_constraints/. Task executor uses go-nvml from NVIDIA to access the gpu stats.

• GPU are exposed to the meca task executor via the docker runtime options, and turn on task executor support via has_gpu in the config yaml.
• Tasks specify use_gpu and gpu_count to request for gpu resources.
• For one GPU, it is allocated in full to a task.
• the /stats handle now report the number of gpu available and the gpu model.