Merge branch 'voronoi' into 'master'

.gitlab-ci.yml

CMakeLists.txt

ci/build_mpi_voronoi.sh

contrib/voro++/.gitignore

contrib/voro++/CMakeLists.txt

contrib/voro++/examples/basic/Makefile

contrib/voro++/examples/basic/README

contrib/voro++/examples/basic/import.cc

contrib/voro++/examples/basic/import.pov

contrib/voro++/examples/basic/pack_ten_cube

contrib/voro++/examples/basic/platonic.cc

contrib/voro++/examples/basic/random_points.cc

contrib/voro++/examples/basic/single_cell.cc

contrib/voro++/examples/custom/Makefile

contrib/voro++/examples/custom/README

contrib/voro++/examples/custom/cell_statistics.cc

contrib/voro++/examples/custom/custom_output.cc

contrib/voro++/examples/custom/custom_output.pl

contrib/voro++/examples/custom/custom_output.pov

contrib/voro++/examples/custom/pack_six_cube