Allow nvisnodes = 1

[sujoy-basak]

Currently, trixi2vtk gives an error when I try to pass nvisnodes = 1. This PR tries to fix it. The nvisnodes = 1 is a useful option in TreeMesh as it allows us to visualize adaptively refined elements (and not solution points).

A visualization with initial refinement level = 2 and nvisnodes = 1 can be found below:

[ranocha]

Thanks a lot!

• Did you check what happens with other mesh types?
• Could you please add a regression test for this feature as well? You can find such tests, e.g., in https://github.com/trixi-framework/Trixi2Vtk.jl/blob/main/test/test_2d.jl. The reference data are created locally first and uploaded to https://github.com/trixi-framework/Trixi2Vtk_reference_files (you can make a PR there). Then, we need to update the reference commit in

  Trixi2Vtk.jl/test/test_trixi2vtk.jl

  Line 46 in 2444ff1

  const TEST_REFERENCE_COMMIT = "7f8c33ef2a0cb2a06470fead8d7bf3b170a7c178"