Update UnsteadyReactionDiffusion solver for explicit time-stepping

Issue/feature addressed

This MR update the UnsteadyReactionDiffusion solver to use explicit time-stepping in addition to IMEX schemes.

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).