Feature/redesign helmholtzmatfree

Issue/feature addressed

• Add HelmholtzAVXSumFac and corresponding unit tests.
• Resolve bugs in all AVXSumFac (aka 'matfree') implementation.

Proposed solution

Implementation

Tests

Suggested reviewers

Notes

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• [ ] Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).