Tidy-up NekNonlinSys and fix some issues

Issue/feature addressed

• Remove unused function SetRefResidual in NekNonlinSys.h
• Replace inappropriate occurrences of boost::lexical_cast<int> by boost::lexical_cast<NekDouble> in NekNonlinSys.cpp
• Remove duplicate duplicate counter variable k in NekNonlinSysNewton.cpp
• Remove uninitialized variable code dependency in CalcInexactNewtonForcing function in NekNonlinSysNewton.cpp
• Modification in NekNonlinSysNewton.cpp to eliminate one superfluous residual evaluation.

Proposed solution

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

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).