Add NekMesh docker image and build configuration

Issue/feature addressed

NekMesh, while being part of Nektar++, can also be used as a stand-alone executable using the Nektar++ library. This MR adds some functionality to make it simpler to build, package and distribute NekMesh when required.

Proposed solution

There are two parts to the proposed solution:

1. Add a NekMesh docker and CI runner. The runner will not be used in this MR but the functionality will be used in the future for NekMesh specific tests.
2. Add a CMake cache file that will load the defaults for building NekMesh without the Nektar++ solvers. This can be called with the -C option in cmake, e.g. cmake ../ -C ../cmake/NekMesh.cmake.

Implementation

Created the files for the docker nekmesh environment and executable and added it to the gitlab CI. Created a cmake cache file with the required NekMesh only default values.

Tests

Brief test added to test functionality of NekMesh in the new image.

Suggested reviewers

Dave, Chris, Jacques, Moshen.

Notes

This MR is more about building the infrastructure for supporting NekMesh than any useable changes.

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).