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

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• All automated tests pass
• Reference related issues https://gitlab.indiscale.com/caosdb/customers/umg/management/-/issues/43
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• 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)
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integrationtests/test_data/extroot/use_case_simple_presentation/cfood.yml

@@ -25,8 +25,8 @@ extroot:
               parents:
                 - mdfile
               role: File
-              path: $DataFile
-              file: $DataFile
+              path: ${DataFile.path}
+              file: ${DataFile.path}
 
             Experiment:
               mdfile: $mdfile
@@ -68,8 +68,8 @@ extroot:
               parents:
                 - mdfile
               role: File
-              path: $DataFile
-              file: $DataFile
+              path: ${DataFile.path}
+              file: ${DataFile.path}
 
             Experiment: {}