Extend Reynolds Stresses filter to passive scalars

Issue/feature addressed

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The FilterReynoldsStresses module can be extended to passive scalars, such as <u theta> in thermal convections. This extension is straightforward.

Proposed solution

A summary of the proposed changes and how they address the issue/feature at hand. Whenever possible, describe alternatives your considered and decisions you made.

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

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).

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)