When the JSON is build to represent an XML tree, sometimes the descendents contain multipe entries of the same type. These should be combined into a common collection.
[x] Add a method: combine to normaliser which combines collections of elements of the same type into the same collection
[x] Add tests to check handling of combine method on abstract elements
When the JSON is build to represent an XML tree, sometimes the descendents contain multipe entries of the same type. These should be combined into a common collection.