feat(tutorial): WeatherServerApp now relays the last received DENM

[hoelger]

Description

The all-knowing weather server sends out warnings from second one. So any cell device will omit the icy region from the beginning on, and not a single cell-car will go towards the icy region.
I think it's more realistic, that the server can just relay the warnings that it received by cars.
For simplicity of the tutorial, the server will just memorize the last warning, and relay this last warning, independent from which car and from where it was sent.

Issue(s) related to this PR

• Resolves internal issue 887

Affected parts of the online documentation

• Changes in the documentation required!

Definition of Done

Prerequisites

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Required

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Requested (can be enforced by maintainers)

• New functionality is covered by unit tests or integration tests. Code coverage must not decrease.
• If a bug has been fixed, a new unit test has been written (beforehand) to prove misbehavior
• There are no new SpotBugs warnings.

Special notes to reviewer

[kschrab]

Please fill the description template. I guess you'd need to update BarnimReleaseIT as well.