Converter from cgns file into NekMesh format

Issue/feature addressed

No reader from CGNS to Nektar format

Proposed solution

Based on the Star and GMsh inputs we have developed a first version of a CGNS reader and added the CGNS library into the ThirdParty distribution

Implementation

As above

Tests

Have added a series of test in standard manner

Suggested reviewers

Dave and Kaloyan

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).