Some tidy-up in CompressibleFlowSolver and LinearAlgebra

Issue/feature addressed

This is an overall tidy-up of CompressibleFlowSolver and LinearAlgebra. Here is a summary of the changes:

• Reorganize CompressibleFlowSystemImplicit.cpp for a more consistent file structure.
• Use default keyword for constructor
• Remove unnecessary SetupNekNonlinSystem accessors and virtual functions v_SetupNekNonlinSystem (see !1709 (merged))
• Remove unused variable NekNonlinSysTolerance" from NekNonlinSys.h(see !1708 (merged))
• Make relevant public functions in CompressibleFlowSystemImplicit.h protected
• Change NewtonAbsoluteIteTol parameter to NewtonRelativeIteTol for consistency
• Remove redundant tolerance limiter in GMRES (see !1707 (merged))
• Simplify code

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).