Insufficient test coverage of grid replacements / non-matching grids

[keileg]

The current set of tests for grid replacement, mainly located in tests/grids/test_mortar_grid.py, but also partly in tests/grids/test_match_grids.py gives insufficient coverage of the actual implementation. The latest case in point was #1088, which was not covered by the tests.

As non-matching grids are not much used (AFAIK), improving the tests is not a high priority task. For the time being this issue mainly serves as a reminder, and as a place to store ideas.

To be considered

Beta Give feedback

1. Grids not aligned with the coordinate axes
2. Grids with a sufficient number of cells to allow errors to show up (what this means is currently not clear to EK, but current tests with a single grid are probably enough - see for instance bug match grids #1088, where a missing transpose could not possibly be caught if the matrix to be transposed was 1x1).
3. Grids where the two sides of the new higher-dimensional grid have non-matching nodes.
4. ??