Dynamic model fetching#10

Enabled the model fetching to be dynamic i.e. only the requested set of models is retrieved which results in a huge time boost.

Extended the testing of apiSlice with mocked api responses (that`s what the 2 json files are for => work with real data that e.g can be transformed, etc.). Func / Line Coverage is now ~90%, Branch ~75%.

Additionally I moved the "Example Group" from the modelSlice into index.js: at the beginning just an action to add the three default models is dispatched. The test store dispatches this also before final creation, so all tests can be performed as usual.