Vor2d mesh faces fix

[brryan]

Background

• The recently added python 2D voronoi generator was not outputting some variables correctly
• It's difficult to debug computational geometry without visualizing it

Purpose of Pull Request

• Fixes re-git issue #1350

Description of changes

• Fix bugs in vor_2d_mesh mesh type with faces that share nodes and boundary data
• Add x3d_plotter.py script that quickly plots 2D x3d mesh files

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Travis/Appveyor CI checks pass
  • Code coverage does not decrease
  • Valgrind test passes
  • Toss3 checks pass
  • Trinitite checks pass
  • Code reviewed/approved, sufficient DbC checks, testing, documentation
  • Enhance src/mesh/python README.md

[brryan]

@KineticTheory were there other details you wanted added to the README? I'm happy to add to it

[RyanWollaeger]

LGTM - just a few questions.

[codecov]

Codecov Report

Merging #1103 (5fe33f3) into develop (a659254) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1103   +/-   ##
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  Coverage     88.7%   88.7%           
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  Files          374     374           
  Lines        18593   18593           
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  Hits         16497   16497           
  Misses        2096    2096