Add region output, with all the cells, to x3d_generator.py.

[RyanWollaeger]

Note: it is planned to eventually permit multiple regions/matids.

Background

• Some dependent packages may need to parse region files corresponding to the matid field in X3D.
• Eventually, x3d_generator.py should support multiple simple regions (i.e. matids), but the intent here is to merely ensure compatibility with parsers.

Purpose of Pull Request

• Add region output, with all the cells, to x3d_generator.py for parsers requiring X3D region files.

Description of changes

• Add region output, with all the cells, to x3d_generator.py.

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

[codecov]

Codecov Report

Merging #1112 (cb40e3d) into develop (3c93de8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1112   +/-   ##
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  Coverage     88.9%   88.9%           
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  Files          376     376           
  Lines        19569   19569           
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  Hits         17416   17416           
  Misses        2153    2153