[!299] Provide fallback for sum factorized geometry mixins if not using sum factorization

Merge branch 'feature/geometry-mixins-fallbacks' into 'master'

ref:extensions/dune-codegen We can disable sum factorization on boundary
integrals. This commit allows us to use a different mixin for those integrals.

See merge request [extensions/dune-codegen!299]

  [extensions/dune-codegen!299]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/299