Support for root file input directly #474

Job	Run time
conda install (ubuntu-20.04)	5m 36s
Build documentation	2m 13s
conda install latest tensorflow (ubuntu-20.04)	14m 52s
Unit tests (ubuntu-20.04, 3.7)	13m 59s
Unit tests (macos-11, 3.7)	27m 25s
Unit tests (ubuntu-20.04, 3.6)	19m 19s
Unit tests (ubuntu-20.04, 3.8)	22m 42s
Test uproot3 (ubuntu-20.04, 3.7)	14m 10s
Style checks	3m 16s
Coverage (ubuntu-20.04, 3.7)	21s
	2h 3m 53s

Provide feedback