Textual representation of the model does not show Data variables or dependency on them

### Description

```python
import pymc as pm
from pymc.printing import str_for_model

with pm.Model() as m:
    x = pm.Data("x", 0)
    y = pm.Normal("y")
    f = pm.Deterministic("f", x + y)
    
str_for_model(m)
# y ~ Normal(0, 1)
# f ~ Deterministic(f(y))
```