bench_plot_fastkmeans.py

* add a title to all benchmark figures/windows
* "bench" is non-standard slang for benchmarking
* remove calls to pyplot.clf because the previous call
  to pyplot.figure sets the current figure
* standardize on "Time (s)" as time axis label
* move some benchmark plot legends to the upper left corner of the plot:
  most benchmarks expand from (0,0) up and to the right
* simplified plot titles and moved constant info from legend into title
* fixed typos
* made some labels more viewer-friendly

Fixes #2023.