go/neo/t/benchplot: New program to visualise neotest benchmarks (draft)

Add the program that reads results from either bench-local or bench-cluster
neotest output and visualizes it. It uses benchlib.py module to read data
in Go benchmark format(*), processes them and plots scalability and other
graphs via matplotlib.

There are lots of hacks and rough edges, and in particular callout coordinate
calculation is completely wrong. However even in this state benchplot was used
to prepare the graphs in http://navytux.spb.ru/~kirr/neo.html and
http://navytux.spb.ru/~kirr/misc/neo·P4.html .

Some draft history related to this patch:

	lab.nexedi.com/kirr/neo/commit/078c9ac3        X move benchlib to -> https://lab.nexedi.com/kirr/pygolang
	lab.nexedi.com/kirr/neo/commit/0edd5129        X benchplot: Teach it to understand benchmark names for partitioned NEO clusters
	lab.nexedi.com/kirr/neo/commit/a1dde3c9        X deco-rio timings
	lab.nexedi.com/kirr/neo/commit/916782b6        X normalize/convert units, so that disk and ping/tcp latencies could be plotted too
	lab.nexedi.com/kirr/neo/commit/f5fec740        X switch node info to labels; start adding that to plot
	lab.nexedi.com/kirr/neo/commit/906462a3        X neotest: Move cluster / node out fro benchmark name to label in environment
	lab.nexedi.com/kirr/neo/commit/cceca65f        X benchplot: Start of automated plotting for neotest benchmark data
	lab.nexedi.com/kirr/neo/commit/a9b10a45        X benchlib/benchstat: Emit label:value info for several labels on one line, similary to go version
	lab.nexedi.com/kirr/neo/commit/502d9477        X benchlib: Python module to read & work with data in Go benchmark format

(*) benchlib.py is now part of pygolang: https://pypi.org/project/pygolang .