Add fix for compiling against tetgen 1.6.0

Issue/feature addressed

Currently we cannot compile against tetgen 1.6 due to an API change.

Proposed solution

TetGen does not provide a version of the library in the header file. A test was added to CMake to check for the presence of the function and an #ifdef is included to call the new function name.

Implementation

See above.

Tests

n/a at present since current CI doesn't include 1.6 to test against, but have checked manually.

Suggested reviewers

@CFD-Xing

Notes

Checklist

• Functions and classes, or changes to them, are documented. - [x] 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).