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
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• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
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src/doc/cfood.rst

@@ -173,8 +173,8 @@ in a vairable with the same name (as it is the case for other Records).
         records:
           fileEntity:
             role: File           # necessary to create a File Entity
-            path: somefile_path  # defines the path in CaosDB
-            file: somefile_path  # path where the file is found locally
+            path: somefile.path  # defines the path in CaosDB
+            file: somefile.path  # path where the file is found locally
           SomeRecord:
             ParameterFile: $fileEntity  # creates a reference to the file