cleaner distance code

[mkborregaard]

I think I edited the code for clarity. It is also like twice as fast for rand on a 1000 x 1000 matrix. Note that this is made against #13

[codecov-io]

Codecov Report

Merging #20 (20bc74f) into feature/DistanceGradient (0904cf2) will increase coverage by 2.09%.
The diff coverage is 0.00%.

@@                     Coverage Diff                      @@
##           feature/DistanceGradient      #20      +/-   ##
============================================================
+ Coverage                     36.36%   38.46%   +2.09%     
============================================================
  Files                             8        8              
  Lines                            55       52       -3     
============================================================
  Hits                             20       20              
+ Misses                           35       32       -3     

Flag	Coverage Δ
unittests	38.46% <0.00%> (+2.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/algorithms/distancegradient.jl	0.00% <0.00%> (ø)

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[tpoisot]

I really need to read the Iterators documentation...