MAINT: centralize the list of numpy functions that map 0 to 0
MAINT: ufunc tuple -> set
PEP8: spacing
MAINT: ufunc set -> frozenset
Merge pull request #4297 from argriffing/consolidate-ufuncs-with-fixed-point-at-zero

MAINT: centralize the list of numpy functions that map 0 to 0