[!346] Make it possible to use coefficient functions in systems

Merge branch 'feature/coefficient-functions-for-systems' into 'master'

ref:extensions/dune-codegen Since DiscreteGridViewFunctions do so far not
support power or composite functions the generator stores a scalar function
for each component.

See merge request [extensions/dune-codegen!346]

  [extensions/dune-codegen!346]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/346