Restructure repository to in a more logical way:

* Code from `test/` goes to `examples/`. This includes the 2D test case and the dummy solver.
* `test/` still contains reference data and scripts to run tests.
* Renamed directories to have more consistent names.
* Removed references to "iterative" and "partitioned" as much as possible.
* Adds 3D example to tests and also removed legacy dependency of 3D case.

See merge request !48
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