Tidy-up ADRSolver

Issue/feature addressed

This is a tidy-up of the ADR solver from the review of the user-guide

• Add provision for diffusion coefficient for the SteadyAdvectionDiffusion and SteadyAdvectionDiffusionReaction solvers
• Remove SteadyDiffusion and SteadyDiffusionReaction from the factory

Some additional tidy-up has also been realized:

• Use default keyword for destructor
• Remove redundant header declaration
• Remove unused member variables

Proposed solution

Implementation

Tests

Suggested reviewers

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Notes

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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).