create save / update / delete / create methods for models

[lukas-h]

• Model.update() / Model.delete() method, if selfRef exists. Implemented like this: Future<void> delete() => selfref.delete(); and Future<void> update() => selfRef.updateData(toMap());

• save/setData method

•  constructor factory Model Model.get(DocumentReference selfRef) => ...

[lukas-h]

implementation:

• extension on Model

• new factory generator