add tests for non-equidistant grids

Many recently added functions ( clipping, regridding, partitioning, masking) are not tested on non-equidistant grids.
We should test what features work and what features don't, and then we can decide what we want to fix and what we won't support for such grids.
If we won't support a feature for non-equidistant structured grids we should add a user warning or error message.
One way to test all this is to use xugrid (through imod-python functionality) to regrid existing equidistant structured test-models to non-equidistant grids (and while at it, to triangles and voronoi cells ). This last step could reduce the number of models in the testbank significantly, because we would create a single set of structured equidistant test-models and use each model 4 times:

1. as is
2. regridded to non-equidistant
3. regridded to triangles
4. regridded to voronoi.

Start by reviewing the existing structured equidistant test-models in the testbank and the clipping, regridding, partitioning, and masking tests. Use xugrid through imod-python functionality to create non-equidistant, triangular, and voronoi variants, then record which features work or fail. Done means the supported cases are covered and unsupported cases produce the required warning or error.