Better support for large data #422

Job	Run time
Unit tests (ubuntu-20.04, 3.7)	17m 49s
Build documentation	2m 28s
conda install latest tensorflow (ubuntu-20.04)	14m 55s
conda install (ubuntu-20.04)	4m 33s
Unit tests (macos-11, 3.7)	22m 43s
Unit tests (ubuntu-20.04, 3.6)	17m 25s
Unit tests (ubuntu-20.04, 3.8)	13m 44s
Test uproot3 (ubuntu-20.04, 3.7)	21m 58s
Style checks	2m 50s
Coverage (ubuntu-20.04, 3.7)	11s
	1h 58m 36s

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