get_dataset_names implemenation/documentation is inconsistent

#200 implemented in #206 added the method `get_dataset_names`

the method is documented as follows in `ABCModelingEngine`:

```
def get_dataset_names(self):
    """ Returns the names of the all the datasets in the engine workspace.
```

But the testing framework, implementation in H5CUDS and in the UML diagrams treats the method as returns an iterator.
