Add a sample module

[JakobMiksch]

fixes #265

• This PR adds a minimal module to app-starter that should help new users to create their custom modules.
• This new SampleModule.vue is extensively commented to explain the basic anatomy of a module
• It contains a simple access of the OpenLayers map to make creating custom map-related modules easier

[fschmenger]

Hi @JakobMiksch, this a first good step in the right direction!

Before you close #265, keep in mind, that we probably need some sort of API documentation for the module-cards. The available generic properties are at least documented on the app-conf level, so maybe this isn't urgent yet.
Also my point from the comment still stands, that we should factor out / separate the examples from the starter templates on the long run.
Maybe you could set up some further issues (1. reusable component doc, 2. app-starter example separation) from that or just keep #265 open?!

[JakobMiksch]

@fschmenger thanks for you comment. I agree with you that we should not forget about the other parts of #265

[JakobMiksch]

I created two new issue #274 and #275
Then #265 can be safely closed

[chrismayer]

Thanks for this. Please merge.