Write mesh type as a dataset always

[pshriwise]

Description

As described in #3249, a change that writes the mesh type as an attribute instead of a data set in the statepoint files along with a corresponding update in the Mesh.from_hdf5 method caused a compatibility issue with statepoint files generated before #3221.

Some of the concrete mesh implementations were also writing the type as a dataset, which is why this wasn't caught in CI. These changes return the mesh type to a dataset, now written only from the Mesh::to_hdf5 method. The Mesh.from_hdf5 method has also been updated accordingly along with the documentation.

I'm open to thoughts on how to test this, but none that are straightforward come to mind immediately.

Thanks for the quick notice @shimwell!

Fixes #3249

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• [] I have added tests that prove my fix is effective or that my feature works (if applicable)