Add a FieldConvert utility to compute the divergence of the velocity

Issue/feature addressed

Add a FieldConvert utility to compute the divergence of the velocity field in order to check conservation of mass for the incompressible Navier-Stokes solver.

Proposed solution

A new utility is added to compute the divergence of the velocity

Implementation

A new header file ProcessVelocityDivergence.h and a new cpp file ProcessVelocityDivergence.cpp are added to the FieldUtils library. The CMake file library/FieldUtils/CMakeLists.txt is also modified

Tests

Notes

Please add any other information that could be useful for reviewers.

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)