Resolution improvement and extra_var in data #410

Job	Run time
conda install (ubuntu-20.04)	5m 10s
Unit tests (ubuntu-20.04, 3.7)	17m 23s
conda install latest tensorflow (ubuntu-20.04)	14m 27s
Build documentation	2m 25s
Test uproot3 (ubuntu-20.04, 3.7)	16m 6s
Unit tests (macos-11, 3.7)	20m 5s
Unit tests (ubuntu-20.04, 3.6)	11m 47s
Unit tests (ubuntu-20.04, 3.8)	10m 47s
Style checks	3m 16s
Coverage (ubuntu-20.04, 3.7)	12s
	1h 41m 38s

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