ENH: add a decorator that adds a path variable

Summary

file system related structure elements can use it to have a variable that can be accessed in the cfood that contains the file system path of the structure element

Also, did some refactoring of the related StructureElements

Focus

The new decorator.

I enhanced the docs quite a bit, but there is still much to do...

Test Environment

Added a unit test. Adopted an integration test

Check List for the Author

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• All automated tests pass
• Reference related issues https://gitlab.indiscale.com/caosdb/customers/umg/management/-/issues/43
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.