Pipeline with more than one `Output` block can be built

Currently the `FunctionalPipelineBuilder` does not check that only one `Output` block per output exists.

For example:

```python
with FunctionalPipelineBuilder() as b:
    X = b.Input('X')()
    b.Output('some_name')(X)
    Y = b.Input('Y')()
    b.Output('some_name')(Y)  # there should be raised exception

b.build()  # or there
```