Save information about structured VTK files

[MuellerSeb]

Since we now got reading routines for structured VTK meshes with PR https://github.com/nschloe/meshio/pull/399, it would be nice to conserve the information about the structured mesh.

Therefor we could add the dataset dict form

https://github.com/nschloe/meshio/blob/07a88dae1a4d8037904074f6f0ac6f20a8bf5c82/meshio/vtk_io.py#L153

to the Mesh container. The dataset dict could be initialized in the Mesh container as:

self.dataset = {"type": "UNSTRUCTURED_GRID"}

to be consistent.

I can come up with a PR to tackle that.

[nschloe]

Perhaps we could then also write structured meshes.

[MuellerSeb]

That would be nice, since the evtk package got some down-sides.

[MuellerSeb]

Perhaps we could then also write structured meshes.

Question is, which other mesh-formats provide structured meshes? And: Are they compatible?

[nschloe]

Yeah, we should probably collect formats which have structured meshes and see what their representations have in common.