Host-wide measures are double-counted in the GUI

[crusaderky]

There are some problems with totalizations in the Bokeh GUI whenever there's more than one worker per host. They are particularly glaring on LocalClusters.

• In the Workers tab:
  • The cluster total for the columns net read, net write, disk read and disk write sums up the value for each worker. However, these are host-wide measures, so if two or more workers sit on the same host, the total will be double-counted.
  • The cluster total for the columns gpu_memory_used and gpu_utilization sums up the value for each worker. However, two workers may share the same GPU, depending on the CUDA_VISIBLE_DEVICES environment variable (see nvml.py). If the variable is not set and in general on single-GPU hosts, all workers on the same host will share the same GPU. Again, this leads to double-counting.
  • The cluster total of the column event_loop_interval is a sum of the workers. This makes no sense; it should be a mean.
• The More... -> Workers Disk and More... -> Workers Network tabs show one bar per worker. This is misleading; there should be one bar per host.
• The More... -> GPU Memory and More... -> GPU Utilization tabs show one bar per worker. This is misleading; there should be one bar per GPU.