Add diagram for data flow in Keanu case study

See merge request !1

Added a minimum example of a "lang" content to make it clear that it doesn't need to be so elaborate.

Issue #3: Reworked campaign configuration: Instead of an "avg" measurement method, which needs to use sub-periods,
we allow measurement periods to be defined so they can be repeated as often as needed.
This simplifies code and data handling.

Issue #3: Don't allow to ommit timezone from timestamps to not complicate parsing and avoid ambiguity.

Issue #3: Instead of defining campaigns in an array, use an object and use their keys as ID. Faster lookup, inherent uniqueness.