SEC-184 - Add common pooled securities statistics

[ElisaKendall]

Signed-off-by: Elisa Kendall ekendall@thematix.com

Description

1. Added a synonym of 'average' to 'arithmetic mean'
2. Moved a number of pool-specific analytical statistics from the provisional debt analytics ontology to the pool-backed securities ontology, including but not limited to: absolute prepayment rate, default rate, weighted average loan age, weighted average remaining maturity, prepayment speed, and weighted average coupon
3. Added single monthly mortality to the set of possible statistics since it was mentioned

Fixes: #1921 / SEC-184

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).