Implement implicit time-discretization in SWE solvers

Issue/feature addressed

• Implement implicit time-stepping (JFNK) for SWE
• Move CopyBoundaryTrace from base class to LinearSWE
• Add const keyword when appropriate
• Remove unused function WallBoundary
• Fix minor inconsistencies between derived classes
• Tidy-up implementation of the NonlinearPeregrine solver

Proposed solution

Implementation

Tests

solvers/ShallowWaterSolver/Tests/NonlinearSWE_Peregrine_SolitaryWave_DG_P6_implicit.xml

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).