make sure broadcasted function runs just once per element

[phaverty]

Fixes #45

[codecov]

Codecov Report

Merging #46 (6e7bfb1) into master (393ae93) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   91.96%   91.90%   -0.06%     
==========================================
  Files          15       15              
  Lines         734      729       -5     
==========================================
- Hits          675      670       -5     
  Misses         59       59              

Impacted Files	Coverage Δ
src/group_generics.jl	83.33% <ø> (-2.39%)	⬇️

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