ITEM_STRUCTURE & ITEM / CLUSTER / ELEMENT

Needs:

• solve the pizza problem - multi-value items
• ITEM_SINGLE, ITEM_LIST etc are not used much in archetyping, and are also not flexible enough because they cannot occur anywhere in a CLUSTER structure
• add a type that is a mixture of CLUSTER and ELEMENT, i.e. has a value and also children, to allow for the fractal nature of data, with a 'summary' value, plus underlying detail

The current models.

Candidate A - make ITEM_STRUCTURE inherit from CLUSTER