feat(bpmn-model) create a BpmnModelInstnace with a copy of the underlying DomDocument

Hi,

I recently had a use case where I needed to create a copy of a ```BpmnModelInstance```, which can be done by converting the instance to a ```String```, creating an ```InputStream``` from it and then reading a ```BpmnModelInstance``` from the stream. I thought that it might be a good idea to have this functionality in the API.

I was not sure whether to put this in the ```BpmnParser```, ```Bpmn``` or some other place, so I put it in the ```Bpmn```. Let me know what you think.