[Task] Create a clear definition of "Collection Aspect"

[atextor]

Is your task related to a problem? Please describe.
There is a customary concept of "Collection Aspect":

• In Best Practices it is mentioned: "If there is exactly one Property in an Aspect that has a Collection, List, Set, Sorted Set or Time Series Characteristic (and no other Properties), the Property should be named items."
• The esmf-sdk generates specific Java classes for Aspects that happen to have exactly one Property with a Set/List/Collection Characteristic

Describe the solution you'd like
Define what a "Collection Aspect" encompasses and what the implications on the JSON payload are.

[aniluede]

Once the Collection Aspect concept is defined, remove quotation marks from "Collection Aspect" in Best Practices section.

Also, refine definition given there and/or link to the new section created that holds the definition. (With current wording, it is not exactly clear whether a Collection Aspect can only have one Property in total or only one Property with certain Characteristics + there can be other Properties.)

[aniluede]

For the part mentioned above – "The esmf-sdk generates specific Java classes for Aspects that happen to have exactly one Property with a Set/List/Collection Characteristic" –, create documentation where this is mentioned/explained.

Then link to this documentation from the "Collection Aspect" chapter in the Best Practices section. (Currently the section does not provide a reason for the naming advice that gets promoted there as a best practice. Therefore, provide this reason and link out to further details in the ESMF SDK docs once this has been created through this very task here.)