Replace boost::thread with std::thread

Issue/feature addressed

This MR replaces boost::thread with std::thread.

Proposed solution

Implementation

The vast majority of boost::thread objects and classes are available under std::thread. There are a few exceptions:

• std::barrier is not available until C++20 but a small Barrier class will suffice for our applications
• BOOST_SPIRIT_THREADSAFE has been removed, but I do not think this is a particular problem since Interpreter objects are not used in a multi-threaded fashion anywhere to the best of my knowledge.

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).