Inject: simplify inject, remove unnecessary code
RefProxyList: list proxy for references

As needed for sources of EntityWithSources
EntityWithSourcesMixin: inject sources list

- Add mixin that enhances all classes that derive from
  EntityWithSources
- Write test for sources list of DataArray
- This fixes #13
Merge pull request #18 from stoewer/with-sources

Implement mixin for EntityWithSources

LGTM