Resolve "setting of [grid]extensions seems to not work with gmsh grids"

What does this MR do?

Avoid reading grid.extensions when loading a GMSH grid from a mesh file.

This is achieved by removing the extensions from the respective model configuration and making sure they are only read during grid creation. Additionally, the docs now mention more explicitly which values are ignored in each of the cases set by grid.gridType.

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• Implemented bugfix
• [ ] Added/Updated tests
• Added/Updated documentation
• Pipeline passing
• [ ] Squash option set Cleaned up history using git rebase
• Delete branch option set
• Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #193 (closed)