Isolate Meshing Routines

[zhang-alvin]

Mandatory Checklist

Please ensure that the following criteria are met:

• [x] Title of pull request describes the changes/features
• [x] Request at least 2 reviewers
• [x] If new files are being added, the files are no larger than 100kB. Post the file sizes.
• [x] Code coverage did not decrease. If this is a bug fix, a test should cover that bug fix. If a new feature is added, a test should be made to cover that feature.
• [ ] New features have appropriate documentation strings (readable by sphinx)
• [x] Contributor has read and agreed with CONTRIBUTING.md and has added themselves to CONTRIBUTORS.md

As a general rule of thumb, try to follow PEP8 guidelines.

Description

This PR aims to isolate the meshing calls from NumericalSolution and put it under a function generateMesh in MeshTools. The result is a more readable initialization in NS_base and makes the mesh generation process more independent of the analysis.

To make this transition possible, a number of meshing specific options need to be defined in the 'domain.MeshOptions' object. This includes attributes like genMesh, nnx, nny, nn, hex, quad, and triangleOptions. This will hopefully make the setup more readable and debuggable.

[codecov[bot]]

Codecov Report

Merging #1238 (012270f) into master (2b9aa83) will increase coverage by 0.07%. The diff coverage is 63.06%.

@@            Coverage Diff             @@
##           master    #1238      +/-   ##
==========================================
+ Coverage   52.74%   52.82%   +0.07%     
==========================================
  Files         531      531              
  Lines      109730   109745      +15     
==========================================
+ Hits        57876    57969      +93     
+ Misses      51854    51776      -78     

Impacted Files	Coverage Δ
proteus/tests/SWFlow/mach_flow.py	0.00% <0.00%> (ø)
proteus/tests/SWFlow/runup_solitaryWave.py	0.00% <0.00%> (ø)
proteus/tests/ci/poisson_3d_c0q1_n.py	93.75% <ø> (ø)
...roteus/tests/solver_tests/import_modules/step2d.py	0.00% <0.00%> (ø)
proteus/MeshTools.py	55.37% <45.53%> (+0.65%)	:arrow_up:
proteus/NumericalSolution.py	78.11% <56.75%> (+7.34%)	:arrow_up:
proteus/tests/elliptic_redist/RDLS/vortex2D.py	86.88% <83.33%> (+1.17%)	:arrow_up:
proteus/tests/elliptic_redist/RDLS3P/vortex2D.py	86.44% <83.33%> (+1.25%)	:arrow_up:
proteus/tests/levelset/vortex2D/vortex2D.py	82.92% <83.33%> (+1.10%)	:arrow_up:
proteus/default_n.py	98.21% <100.00%> (+0.01%)	:arrow_up:
... and 41 more

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