Support for MED multiple meshes

[LTMeyer]

Dear all,

I am having issues to read the following .med mesh file results.med.zip. I get the following error due to the lines https://github.com/nschloe/meshio/blob/main/src/meshio/med/_med.py#L43-L44 :

ReadError: Must only contain exactly 1 mesh, found 2.

The mesh indeed contains two parts that are Boundary and Fluid domain. I understand from this comment #1058 (comment) that multiple meshes are not yet supported. However, in #175 was discussed the implementation of region, which I believe would cover my case of having in domain and boundary regions as separate meshes.

How could multiple meshes in one med files be handled by meshio?

[nschloe]

This relates to an old bug: #268.