Improve stats gathering of mapping-tester

[fsimonis]

Main changes of this PR

This PR improves the gathering of stats in the mapping-tester by:

1. Making it more robust. Various files are now only loaded if they exist. So, post-processing is now optional.
2. Loads cases in parallel, which can help to improve the import speed of events via precice-profiling

Author's checklist

• I used the pre-commit hook and used pre-commit run --all to apply all available hooks.
• I added a test to cover the proposed changes in our test suite.
• I updated the documentation in docs/README.md.
• I added a changelog entry in ./changelog-entries/ (create if necessary).
• I updated potential breaking changes in the tutorial precice/tutorials/aste-turbine.