Replace deprecated boost::filesystem function calls in Nekmesh library

Issue/feature addressed

The boost::filesystem::extension(fname) function call is deprecated and causes warnings during compilation.

Proposed solution

Use updated boost::filesystem::path(fname).extension() function call.

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

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).