Add random Voronoi mesh to mesh generators

[brryan]

Background

• We want to be able to quickly spin up 2D unstructured meshes that effectively sample different zone connectedness for benchmarking algorithms on unstructured meshes

Purpose of Pull Request

• detail 1, refs Enable travis CI #2 (github number)
• Fixes re-git issue #1350

Description of changes

• Add random Voronoi mesh generator to mesh/python/mesh_types.py
• Modify mesh/python/x3d_generator.py to support more complex mesh case (e.g. differing numbers of nodes per boundary)

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

[RyanWollaeger]

LGTM! Just a few minor questions/comments...

[RyanWollaeger]

What was the issue that precluded differing number of nodes per boundary?

[codecov]

Codecov Report

Merging #1098 (4bc5622) into develop (b366af7) will increase coverage by 2.9%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop   #1098     +/-   ##
=========================================
+ Coverage     85.7%   88.7%   +2.9%     
=========================================
  Files          356     374     +18     
  Lines        17227   18593   +1366     
=========================================
+ Hits         14776   16497   +1721     
+ Misses        2451    2096    -355     

[RyanWollaeger]

LGTM