CADfix developments in NekMesh
This MR encompasses a few separate, but related, features connected to the work with ITI:
- Bidirectional BL splitting (3D only)
- Export mesh to CADfix: all nodes are exported but order of element only 1 or 2
- Force VarOpti to single thread with CFI
Note that the output to CADfix has to be done in the same call as the import from it so that the CAD system/information is intact.