Add custom element type support for `vtk2trixi`

[LasNikas]

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 89.60%. Comparing base (32679eb) to head (cac637d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #991   +/-   ##
=======================================
  Coverage   89.60%   89.60%           
=======================================
  Files         120      120           
  Lines        8588     8589    +1     
=======================================
+ Hits         7695     7696    +1     
  Misses        893      893           

Flag	Coverage Δ
total	89.60% <100.00%> (+<0.01%)	:arrow_up:
unit	64.84% <100.00%> (+<0.01%)	:arrow_up:

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