Small feature to add spatially constant variable diffusion for explicit DG

Issue/feature addressed

This MR adds spatially-constant, but variable, diffusion coefficients to the ADRSolver's UnsteadySystem class.

Proposed solution

The GetFluxVector has been changed to align with the diffusion coefficients.

Implementation

n/a

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).