Merge pull request #144 from jiangyi15/asym_err_combine #636

Job	Run time
conda install latest tensorflow (ubuntu-20.04)	4m 40s
Build documentation	2m 57s
Unit tests (ubuntu-20.04, 3.7)	15m 53s
conda install (ubuntu-20.04)	2m 34s
Test uproot3 (ubuntu-20.04, 3.7)	16m 11s
Unit tests (macos-11, 3.7)	39m 27s
Style checks	2m 17s
Unit tests (ubuntu-20.04, 3.6)	15m 9s
Unit tests (ubuntu-20.04, 3.8)	15m 12s
Coverage (ubuntu-20.04, 3.7)	13s
	1h 54m 33s

Provide feedback