Issue 135 metadata visualization

[JDTheRipperPC]

Resolves #135

Add metadata visualization:

plot: generate a plot using graphviz, if a path is provided save the output into a file.
__str__: added string visualization.

Added graphviz>=0.13.2 to the requirements.

[codecov-io]

Codecov Report

Merging #148 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   87.94%   87.96%   +0.01%     
==========================================
  Files          12       12              
  Lines         863      864       +1     
==========================================
+ Hits          759      760       +1     
  Misses        104      104

Impacted Files	Coverage Δ
sdv/models/copulas.py	100% <100%> (ø)	⬆️
sdv/sdv.py	97.5% <100%> (+0.06%)	⬆️

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