Error when creating mesh from dfsu3d geometry

[jsmariegaard]

Discussed in #391

^{Originally posted by lezhongwen July 21, 2022}
Hello developers, this library has helped me solve a lot of problems recently, thank you for your contributions. I had some problems with reading and writing files.

1. The mesh data extracted from DFSU 3D files cannot be written to a mesh file, and i got ValueError: All elements must have 3 or 4 nodes. Element number 1 has 6 nodes.
2. After I wrote the regular mesh file(it can correspond to dfsu 2d) and the dataset extracted from DFSU 3D into a new DFSU file, the data became DFSU 2D. After checking, I found that not only the data quantity has decreased, but also the geometry attribute has changed.
   I would be grateful for your reply~