Rewrite Expressions such that they may be defined over mixed elements

python/dune/perftool/loopy/transformer.py

python/dune/perftool/pdelab/driver.py

python/dune/perftool/pdelab/parameter.py

python/dune/perftool/ufl/execution.py

...sson/dimension-grid-variations/poisson_1d_cg_interval.ufl

...sson/dimension-grid-variations/poisson_1d_dg_interval.ufl

...dimension-grid-variations/poisson_2d_cg_quadrilateral.ufl

...dimension-grid-variations/poisson_2d_dg_quadrilateral.ufl

...on/dimension-grid-variations/poisson_3d_cg_hexahedron.ufl

...n/dimension-grid-variations/poisson_3d_cg_tetrahedron.ufl

...on/dimension-grid-variations/poisson_3d_dg_hexahedron.ufl

...n/dimension-grid-variations/poisson_3d_dg_tetrahedron.ufl

test/stokes/stokes.ufl

test/stokes/stokes_dg.ufl