ENH: Add simplex tools

[cc7768]

Based on the QuantEcon.py code from QuantEcon/QuantEcon.py#344

Also would close #178

[codecov-io]

Codecov Report

Merging #183 into master will increase coverage by 0.15%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
+ Coverage   90.54%   90.69%   +0.15%     
==========================================
  Files          24       24              
  Lines        1607     1633      +26     
==========================================
+ Hits         1455     1481      +26     
  Misses        152      152

Impacted Files	Coverage Δ
src/util.jl	95.65% <96.15%> (+2.62%)	⬆️

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[oyamad]

Thanks, the code looks good to me, except that test_util is missing in runtests.jl.