Fix Wunused-parameter warnings

Issue/feature addressed

There are a lot of unused parameters, notably in virtual functions. Those unused parameters are not detected on the CI and produced Wunused-parameter warning when the -Wall -Wextra flags are used.

Proposed solution

Using boost::ignore_unused() in problematic functions to eliminate Wunused-parameter warnings.

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).

Warning

On the 19.07 the code formatting (code style) was standardised using clang-format, over the whole Nektar++ code. This means changes in your branch will conflict with formatting changes on the master branch. To resolve these conflicts , see #295 (closed)