Add GPU memory usage to dask_memusage plugin.

[jcfaracco]

Today, it is almost impossible to monitor GPU usage using dask_memusage. Only regular memory is possible.

With the proposed commits it is possible at least to compare the minimum GPU usage with the memory pick using GPUtil library.

The only problem with this approach is that we cannot track memory usage by each process by using nvidia-smi pmon -c 1 -s m for example. For some reason, this command does not work inside docker containers. It is possible to monitor only from the host system.