add tet() functions for VTK

[vincentxwang]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 82.45614% with 10 lines in your changes missing coverage. Please review.

Project coverage is 95.38%. Comparing base (2a0fa81) to head (fbabfcd).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mesh/vtk_helper.jl	82.45%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
- Coverage   96.03%   95.38%   -0.65%     
==========================================
  Files          28       28              
  Lines        3831     3878      +47     
==========================================
+ Hits         3679     3699      +20     
- Misses        152      179      +27     

Flag	Coverage Δ
	95.38% <82.45%> (?)

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