Add dumux-customised checkpointing feature

[Fujikawas]

Description

To close #56.
Different initialization option for checkpointing is provided to match the need of different cases in examples/ and the heat-conduction case .

Checklist

• I made sure that the source files are formatted properly.
• I added my changes to the changelog (CHANGELOG.md)
• I updated the documentation.
• I added a test for the new feature.

[IshaanDesai]

Thanks for this important contribution. I have a few comments (more like questions for my own understanding). I would recommend moving the test from the examples/ folder to a separate test/ folder.

[Fujikawas]

I agree that examples/ folder is not a place for the test. I suggest doing a refactor/renaming after #52, since then we can have proper test/ folder for testing the adapter features.

In my opinion, #52 is independent of creating a test/ folder right now. Are you using the folder hierarchy in examples/ in the build process of the test? If yes, that would be a valid reason to put the test there. We can move the test after finalizing this PR too.

[Fujikawas]

In my opinion, #52 is independent of creating a test/ folder right now. Are you using the folder hierarchy in examples/ in the build process of the test? If yes, that would be a valid reason to put the test there. We can move the test after finalizing this PR too.

The current test/ folder contains scripts to run the examples and is not for tests. Let's do a refactor of the folder structure later.

[IshaanDesai]

The two-scale heat conduction tutorial case with changes from precice/tutorials#676 fails with the changes from this branch. The Newton solver on the macro scale reaches the default maximum permissible iterations (10). This hints at potential issues related to checkpointing the time variables.

[IshaanDesai]

precice/tutorials#676 (comment) shows that the changes here work as expected.

[mathiskelm]

Some comments, otherwise good. Should check again with the tutorials branch.