Feature/tidy up linsys

Issue/feature addressed

This is just a minor tidy-up of the LinSys class and subclasses. Here is a summary of the changes:

• Remove unused variable min_resid in NekLinSysIterCG.cpp.
• Remove redundant counter variable k in NekLinSysIterCG.cpp.
• Remove unnecessary boost::ignore_unused function call in several files.
• Change Array<OneD, NekDouble> vExchange(1, 0.0) to NekDouble vExchange(0.0) in NekLinSysIter.cpp.
• Remove unnecessary lvalue assignment in NekVector constructor call in NekLinSysIterCG.cpp.
• Fix some documentation errors/typos in library/LibUtilities/BasicUtils/Vmath.hpp
• Fix some documentation errors/typos in library/LibUtilities/BasicUtils/Vmath.cpp
• Fix some documentation errors/typos in library/LibUtilities/BasicUtils/VmathArray.hpp
• Fix some documentation errors/typos in library/LibUtilities/BasicUtils/VmathSIMD.hpp

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).