Merge branch 'feature/sumfact-poisson-3d' into 'master'

python/dune/perftool/loopy/buffer.py

python/dune/perftool/sumfact/basis.py

python/dune/perftool/sumfact/sumfact.py

test/sumfact/mass/CMakeLists.txt

test/sumfact/mass/mass_3d.mini

test/sumfact/mass/mass_3d.ufl

test/sumfact/poisson/CMakeLists.txt

test/sumfact/poisson/poisson_2d_order1.mini

test/sumfact/poisson/poisson_2d_order1.ufl

test/sumfact/poisson/poisson_2d_order2.mini

test/sumfact/poisson/poisson_2d_order2.ufl

test/sumfact/poisson/poisson_3d_order1.mini

test/sumfact/poisson/poisson_3d_order1.ufl

test/sumfact/poisson/poisson_3d_order2.mini

test/sumfact/poisson/poisson_3d_order2.ufl