Update solvers' CMakeLists.txt to fix some warnings detection issue

Issue/feature addressed

The current implementation of NektarCommon.cmake does not properly set compiler flags for files in the solvers/ directory. This results in many undetected warnings/errors in the CI.

Proposed solution

The proposed solution is to use modify the solvers' CMakeLists.txt file.

Implementation

A SET_COMMON_PROPERTIES(${name}-obj) is added in the solvers' CMakeLists.txt file.

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)