Symmetric hexagonal stencil class for convoluting tri meshes

[andy-beer]

This change adds a TriMeshStencil class which can be used to smooth (or apply other convolutions) to TriMesh z values.

[andy-beer]

unit tests not yet added

[codecov]

Codecov Report

Attention: Patch coverage is 89.01099% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 82.95%. Comparing base (4d71c84) to head (d5904a0).

❗ Current head d5904a0 differs from pull request most recent head edaffcc. Consider uploading reports for the commit edaffcc to get more accurate results

Files	Patch %	Lines
resqpy/surface/_tri_mesh_stencil.py	88.95%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #788      +/-   ##
==========================================
+ Coverage   82.91%   82.95%   +0.03%     
==========================================
  Files         190      191       +1     
  Lines       32520    32702     +182     
==========================================
+ Hits        26965    27128     +163     
- Misses       5555     5574      +19     

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