Initial mu configurability

[lukelowry]

Description

This is an initial, intentionally small change to make the CommonMath smoothing scale runtime-configurable. PhasorDynamics studies can now set mu ($\mu$) in a .solver.json file, allowing parameter studies without rebuilding GridKit for every value.

Proposed changes

• Add an optional mu with the existing 240.0 default
• Require mu to be positive and finite
• Apply it before model construction and contingency workers
• Document the setting and add runtime behavior coverage

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• N/A The CHANGELOG.md has been updated to reflect the changes.

Further comments

This is not the best long-term API for configuring $\mu$ at runtime, but without this, the results of our paper are very hard to reproduce without a development environment. For that reason, I would like to have this in before we do the first release.

[nkoukpaizan]

I don't think we can merge this, even as a temporary solution. The provenance of the value used will be very hard to establish.

[lukelowry]

I cannot think of an alternative that would be a small change. I would love to have this for the purpose of the paper, but I see the issue.

Unless you have any ideas on how to do this safely, I will close this

[nkoukpaizan]

The ideas I have require widespread changes. We can close for now and keep tracking in #504.