Prepare release 4.26 - Githubissues

[x] run unit tests in dev
[x] update version to "4.26" 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 main and push
[x] create libhxl PyPi release: pip install twine && python setup.py sdist && twine upload dist/libhxl-4.26*
[x] wait until the new release is visible on PyPi https://pypi.org/project/libhxl/4.26/
[x] add git tag "4.26"
[x] create GitHub release

HXLStandard / libhxl-python