Additional fixes for LinearRepn

[jsiirola]

Fixes # .

Summary/Motivation:

This is a follow-up to #2863 to address a few additional errors in the LinearRepn (and to expand the suite of unit tests.

Changes proposed in this PR:

• Resolve additional errors in LinearRepn and _register_new_before_child_dispatcher()
• Expand unit tests

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.