Merge branch 'feature/more-codegen-time-evaluation' into 'master'

ref:extensions/dune-codegen (Partial) Evaluation of arithmetic expressions at
code generation time is a vital tool to generate minimal-FLOP code and a
genuine advantage of code generation. This MR adds evaluations for matrix
inverses, sums and products.

See merge request [extensions/dune-codegen!331]

  [extensions/dune-codegen!331]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/331