Redesign of AUSM Riemann solver class

Issue/feature addressed

The M1Function, M2Function, M3Function, M4Function, and P5Function are duplicated across all AUSM Riemann solver classes.

Proposed solution

Use AUSM0 as a base class for AUSM1, AUSM2, and AUSM3 subclasses.

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Please add any other information that could be useful for reviewers.

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).