Add integral filter to compute the time-dependent evolution of the integral of a field.

Issue/feature addressed

Need to be able to generate a time-dependent integral of the composites during the simulation.

Proposed solution

Add a new filter to calculate the integral over one or more composite groups.

Implementation

New filter named FilterIntegral is added to library/SolverUtils/Filters.

Tests

No test has been added yet, since the tester does not currently have a means to check the contents of a file generated during execution of the test.

Notes

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)