Bugfix to dataset property system (prop_get)
Update graph readers.

- Add some more readers
- Remove yaml, which has an external dependency.
Add tests for different networkx readers
bugfix: for tests, empty database causes new_ds_id to fail
cleanup: no. of nodes and no. of edges now integers
Update settings
docstring parsing: "..." in its own paragraph ends shown part.

- Some docstrings have extra information that should not be shown to a
  jako user.
- Docstrings are only shown up to the first "..." in its own
  paragraph, and the rest is not shown.
Add NullCD method