Create release 4.24

[davidmegginson]

• [x] run unit tests in dev
• [x] update version to "4.24" in setup.py and hxl/__init__.py
• [x] check CHANGELOG
• [x] install and test in clean virtualenv
• [x] commit changes
• [x] merge to test and push
• [x] merge to master and push
• [x] create libhxl PyPi release: pip install twine && python setup.py sdist && twine upload dist/libhxl-4.24*
• [x] wait until the new release is visible on PyPi https://pypi.org/project/libhxl/4.24/
• [x] add git tag "4.24"
• [x] create GitHub release