Restructure repo

Description

Aims to 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.

Checklist

• I made sure that the source files are formatted properly.
• I added my changes to the changelog (CHANGELOG.md)