correctly set up two merged boxes for nonzero Y origin

[jdannberg]

The computation of the upper box origin in the two merged boxes geometry model did not take into account a nonzero origin. This is fixed in this pull request. I also created a test case.

Below are images for a box with a lithosphere thickness of 80 km (the same model as in the new test).

Before:

After:

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[gassmoeller]

Looks good, but can you add a changelog entry for this change? It is an important enough fix to document.

[jdannberg]

Thank you! I added a changelog entry.

[gassmoeller]

/rebuild