Tidy-up TimeIntegration

Issue/feature addressed

Tidy-up TimeIntegration module

Proposed solution

• Remove unnecessary header files
• Use constexpr when applicable
• Consistently specify explicit and implicit derivative variables
• Remove commented code
• Consistently add LUE declspec to member functions
• Remove unused constructor in TimeIntegrationSolutionGLM.cpp
• Remove unnecessary/misleading comment
• Add a flag to check if solution variable is properly updated for multistep approaches

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).