perf: Improve performance by reducing redundant pynvml.nvmlInit call

### perf: Call nvmlInit() and nvmlShutdown() only once (fixes #54) Lots of time is spent on nvmlInit() and nvmlShutdown() for each new_query call. When running in a loop mode (-i), we do not need to initialize and shutdown the nvml library because nvml APIs will be used throughout the lifespan of the gpustat process. Upon importing `gpustat.pynvml`, nvmlInit() will always be called.