Merge branch 'release-3.1.1'

CHANGELOG

CMakeLists.txt

container/Dockerfile

container/Singularity

doc/conf.py.in

doc/container/index.rst

doc/container/singularity.rst

doc/gettingstarted.rst

doc/html/_sources/actions/index.rst.txt

doc/html/_sources/actions/index_dev.rst.txt

doc/html/_sources/buildsystem.rst.txt

doc/html/_sources/changelog.rst.txt

doc/html/_sources/cmake/index.rst.txt

doc/html/_sources/container/docker.rst.txt

doc/html/_sources/container/index.rst.txt

doc/html/_sources/container/singularity.rst.txt

doc/html/_sources/contributing.rst.txt

doc/html/_sources/core/geometry.rst.txt

doc/html/_sources/core/graph_minimizer.rst.txt

doc/html/_sources/core/helper.rst.txt