Use modern CI script

See merge request agnumpde/dune-matrix-vector!6

This introduces the same CI script also used in the other agnumpde repositories.

See https://gitlab.dune-project.org/core/dune-common/issues/77

Feature/organize traits

See merge request !5

... when it does not depend on the function template parameter
(experienced with clang).

 - smoothen docu
 - name helper struct
 - add implementation namespace

The former recursion anchor of the Helper was hardcoded to be
FieldVectors. Now, all non-scalar types are treated hierarchically
and the actual action is only performed on scalars.