Some various tidy-up

Issue/feature addressed

This is just some tidy-up of the code base

Proposed solution

• Correct some typos in comment and/or in warning/error message
• Use c++ literal, when appropriate
• Remove some unused functions
• Remove commented code
• Move MFFDiffusion.cpp to the EquationSystems directory
• Update documentation for max, min, fmax, and fmin mathematical functions.

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).