PreProcessor for AmplitudeModel #425

Job	Run time
conda install (ubuntu-20.04)	5m 40s
Unit tests (ubuntu-20.04, 3.7)	15m 34s
conda install latest tensorflow (ubuntu-20.04)	20m 13s
Build documentation	2m 25s
Unit tests (macos-11, 3.7)	27m 6s
Unit tests (ubuntu-20.04, 3.6)	17m 37s
Unit tests (ubuntu-20.04, 3.8)	15m 44s
Test uproot3 (ubuntu-20.04, 3.7)	20m 43s
Style checks	2m 37s
Coverage (ubuntu-20.04, 3.7)	16s
	2h 7m 55s

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