Some tidy-up in MatrixFreeOps

Issue/feature addressed

Some tidy-up in the MatrixFreeOps library

Proposed solution

• Delete unused file IProductKernelsImp.hpp
• Delete unused function Ndof
• Delete unused overloaded version of PhysDerivTensor2DKernel
• Rename Helmholtz2DKernel as DiffusionCoeff2DKernel
• Rename Helmholtz3DKernel as DiffusionCoeff3DKernel
• Remove commented code
• Use default key word when appropriate.

Implementation

A more detailed description of the changes. Please include any details necessary for reviewers to understand your work, and point out particular points would like them to pay particular attention to.

Tests

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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).