Add RNG to make deterministic

[skygering]

Solution to #34

My pull request to VoronoiDelauny was merged. My PR allows us to seed the call to random! that was causing the non-deterministic behavior, as the different orders of operations were leading to differences in the floating point answer.

Here I have made an optional RNG argument to the call to voronoicells. My tests show that if you provide RNGs with different seeds, voronoicells gives two different answers, and that if we provide the same seed we get the same answer.

This requirers VoronoiDelauny v0.4.4, which I changed in the Project.toml.

[codecov]

Codecov Report

Merging #35 (4b7cfd4) into master (10dd2b9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   89.38%   89.38%           
=======================================
  Files           7        7           
  Lines         226      226           
=======================================
  Hits          202      202           
  Misses         24       24           

Impacted Files	Coverage Δ
src/Cells.jl	88.52% <100.00%> (ø)

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[Wikunia]

Would be great to add this to the readme as I think a lot of developers will expect a deterministic behaviour if not mentioned otherwise.