Transfer of repository information

As far as I understood, `repository` should return the actual used terminology-location (uri/url) of the odML object. Currently these attributes/functions behave in the following way (note: the code example is based on the example in #74):

```python
# for Documents
doc1.repository
# output: 'http://portal.g-node.org/odml/terminologies/v1.0/terminologies.xml'
# BUT doc2 which is the fetched terminology using the repo stated in doc1 does not include the repository attribute (the same holds for sections retrieved from a terminology)
doc2.repository
# output: nothing
```

The used repository information should be automatically transferred retrieved objects.