Add IO graph distribution 'ROOT', which assigns all nodes + edges to … #410

Job	Run time
build (Clang, clang, clang++, Debug)	4m 0s
build (Clang, clang, clang++, Release)	1m 56s
build (GNU, gcc, g++, Debug)	3m 52s
build (GNU, gcc, g++, Release)	2m 14s
	12m 2s

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