Add VTK support for cases where rd.N is a tuple with all equal values

[tristanmontoya]

This is needed for visualizing prism meshes with trixi-framework/Trixi2Vtk.jl#103, which calls SUD_to_vtk_order.

[jlchan]

Woah, sorry I missed this for so long! Let me just run CI and if it works I'll merge immediately.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.05%. Comparing base (86aa5ab) to head (5c6dd7b).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mesh/vtk_helper.jl	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
- Coverage   95.94%   95.05%   -0.89%     
==========================================
  Files          28       28              
  Lines        3846     3926      +80     
==========================================
+ Hits         3690     3732      +42     
- Misses        156      194      +38     

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