Additional ways of defining model relationships

[safo6m]

So far, propertyClass property of Attribute, HeaderAttribute, and HasOne decorators accepted classes only. This pull request introduces a new way of defining relationships by provided string identificators.
In order to use model's string identificator, the said identificator must be set on the model (modelType property), and the model must be provided in the list of models present in the DatastoreService (modelTypes property).

Why would you need to use string identificator instead of the exact class?
In case when two models have references between each other, the reference on one of the sides must be defined via string identificators to break the circular dependency.

https://github.com/infinum/ngx-hal/wiki/DefiningModelRelationships#circular-dependencies