Support factor for model using helicity directly #501

Job	Run time
Unit tests (ubuntu-20.04, 3.7)	17m 54s
conda install (ubuntu-20.04)	4m 45s
Unit tests (macos-11, 3.7)	27m 18s
Unit tests (ubuntu-20.04, 3.6)	16m 56s
Unit tests (ubuntu-20.04, 3.8)	23m 6s
conda install latest tensorflow (ubuntu-20.04)	18m 0s
Test uproot3 (ubuntu-20.04, 3.7)	20m 59s
Build documentation	3m 15s
Style checks	2m 31s
Coverage (ubuntu-20.04, 3.7)	14s
	2h 14m 58s

Provide feedback