Remove 'none' as an 'aggregation' option for event detection and make…

[james-d-brown]

… the 'operation' required when an explicit 'combination' is declared, #423.

Description

Fixes #423.

Changes Made

Testing

Related Tickets

• Fixes [Ticket number]

Checklists

Deployment Plan (For developer use)

How does the changes affect the product?

• Feature addition
• Bug fix
• Database change

Issuer Checklist (For developer use)

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• Informative and human-readable title, using the format: [_pt] PR: <description>
• Links are provided if this PR resolves an issue, or depends on another other PR
• The feature branch you're submitting as a PR is up to date
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• No Placeholder code
• Reviewers requested
• Add yourself as an assignee in the PR

Merge Checklist (For Technical Lead use only)

• If applicable, update README with major alterations