eliminated unnecessary function evals in muller

[jacobwilliams]

Fixes #221

[codecov]

Codecov Report

Merging #222 (d6136f8) into master (1eece9a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   85.02%   85.04%   +0.02%     
==========================================
  Files           9        9              
  Lines        1896     1899       +3     
==========================================
+ Hits         1612     1615       +3     
  Misses        284      284              

Impacted Files	Coverage Δ
src/simple.jl	91.56% <100.00%> (+0.15%)	⬆️

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[jverzani]

Thanks again