FBC-284 - The definition of Issuer should be adjusted to allow the same issuer to issue more than one

[ElisaKendall]

Description

Better integrated contract principal and counterparty with the overall contract pattern; augmented restrictions on issuer to say that they must issue a contract and revised the definition per notes.

Fixes: #2013 / FBC-284

Checklist:

• [x] I'm familiar with the FIBO developer quide. My contribution meets all the requirements described there.
• [x] My contribution follows the principles of best practices for FIBO.
• [x] My changes have been reconciled with latest master and no merge conflicts remain.
• [x] This PR is related to exactly one issue. The issue is referenced by using a GitHub keyword such as "fixes", "closes", or "resolves".
• [x] Hygiene tests have been applied by a PR with "(WIP)" in title.
• [x] The issue has been tested locally using a reasoner (for ontology changes).