New 97-line test file for postprocessing functionality:
- Tests postprocessing fields (strain, stress, reaction force, contact pressure)
- Tests contact problem with two blocks (upper and lower)
- Tests XDMF output writer
- Tests field extraction at specific points
- Tests nodal vector extraction
- Validates contact interface displacement uniformity
Tests complete postprocessing workflow including field computation,
output writing, and field querying at arbitrary points.
Jukka Aho