Increasing output PLY file

[jackylovechina]

For better rendering, output PLY file is added

[codecov]

Codecov Report

Merging #67 (384b304) into master (b0c4ab6) will decrease coverage by 6.56%.
The diff coverage is 2.00%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   76.40%   69.84%   -6.57%     
==========================================
  Files           3        3              
  Lines         517      567      +50     
  Branches       98      102       +4     
==========================================
+ Hits          395      396       +1     
- Misses         85      134      +49     
  Partials       37       37              

Impacted Files	Coverage Δ
engine/mpm_solver.py	70.21% <2.00%> (-8.13%)	⬇️

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