Resolution improvement and extra_var in data #409

Job	Run time
conda install (ubuntu-20.04)	3m 44s
Unit tests (ubuntu-20.04, 3.7)	18m 54s
conda install latest tensorflow (ubuntu-20.04)	17m 34s
Unit tests (macos-11, 3.7)	19m 31s
Unit tests (ubuntu-20.04, 3.6)	11m 23s
Unit tests (ubuntu-20.04, 3.8)	11m 15s
Build documentation	2m 35s
Test uproot3 (ubuntu-20.04, 3.7)	12m 55s
Style checks	2m 35s
Coverage (ubuntu-20.04, 3.7)	17s
	1h 40m 43s

Provide feedback