Changes and Information

• added ccache to CI linux builds (other builds maybe added later after some observation)
• simplified linux build options, removed unused options to make the cache key easier

To see that the cache is working correctly:

• The ccache action reports loading the cache at the beginning of the build job and cache misses/hits at the end
• stored cache files can be viewed in github -> actions -> caches; this development branch doesn't save caches anymore but there are some cache files around from my testing
• I tested the cache on the development branch before I added the restriction to only save on the main branch. compile times went down from ~15 mins to ~5 mins. this is with identical code, i.e. 100% cache hits. Compile times should be checked after a few nightly builds to see if everything works there.

Closes #305

Merge Request - Guideline Checklist

Please check our git workflow. Use the draft feature if the Pull Request is not yet ready to review.

Checks by code author

• Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
• New code adheres to coding guidelines
• No large data files have been added (files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• Tests are added for new functionality and a local test run was successful (with and without OpenMP)
• Appropriate documentation for new functionality has been added (Doxygen in the code and Markdown files if necessary)
• Proper attention to licenses, especially no new third-party software with conflicting license has been added
• (For ABM development) Checked benchmark results and ran and posted a local test above from before and after development to ensure performance is monitored.

Checks by code reviewer(s)

• Corresponding issue(s) is/are linked and addressed
• Code is clean of development artifacts (no deactivated or commented code lines, no debugging printouts, etc.)
• Appropriate unit tests have been added, CI passes, code coverage and performance is acceptable (did not decrease)
• No large data files added in the whole history of commits(files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• On merge, add 2-5 lines with the changes (main added features, changed items, or corrected bugs) to the merge-commit-message. This can be taken from the briefly-list-the-changes above (best case) or the separate commit messages (worst case).