Group imports by StdExternalCrate

[sophie-cluml]

To improve our code's readability, we are using group imports by StdExternalCrate. Across all our modules, the following steps are needed:

• Modify the code with this command: cargo fmt -- --config group_imports=StdExternalCrate
• Add --config group_imports=StdExternalCrate to the CI process like: cargo fmt -- --check --config group_imports=StdExternalCrate
• Submit a PR including the above changes