Add missing override keyword to virtual functions in solvers

Issue/feature addressed

Many virtual functions in derived class of the solvers directory do not have the override keyword. This make the code unsafe if someone changes the definition of the base class.

Proposed solution

Add the override keyword to the relevant functions in the solvers directory. The virtual keyword is also explicitly added to all virtual function as well as the v_ prefix. Some virtual functions have been made non-virtual.

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)