Add utility function for save_as_dot

[RenatoGeh]

This patch simply adds a method to save_as_dot if given a root node Prob\DeltaNode instead of a Vector Prob\Delta.

[codecov-commenter]

Codecov Report

Merging #13 into master will decrease coverage by 0.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   67.26%   67.11%   -0.16%     
==========================================
  Files          20       20              
  Lines        1326     1329       +3     
==========================================
  Hits          892      892              
- Misses        434      437       +3     

Impacted Files	Coverage Δ
src/IO/CircuitSaver.jl	82.00% <0.00%> (-2.54%)	⬇️

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