SEP 051 - Rename TopLevel to AggregateRoot

[jamesamcl]

Since SBOL 1.0, SBOL has had the concept of a "top level", defined in the SBOL 3.0 specification as objects that are not nested inside any other object via composite aggregation. While the term "top level" is an SBOL invention, the classification of such objects is already established in software engineering, particularly in the context of domain-driven design where they are termed "aggregate roots". This SEP proposes the term "aggregate root" is adopted in place of "top level".

Full SEP: https://github.com/SynBioDex/SEPs/blob/master/sep_051.md

[jamesamcl]

Withdrawn, as SBOL3 top levels do not strictly follow the rule "Any references from outside the aggregate should only go to the aggregate root."