Add DataGraph.rectangular() and redefine DataGraph.square() as a convenience wrapper

[alessandro-santini]

The current DataGraph.square() implementation should be generalized and renamed to DataGraph.rectangular().

DataGraph.square() should remain available as the square-specific convenience method, implemented by calling DataGraph.rectangular() with equal side lengths. This should follow the same pattern used in [register.py](https://github.com/pasqal-io/qoolqit/blob/main/qoolqit/register.py).

Proposed changes

• Rename/generalize the current DataGraph.square() implementation as DataGraph.rectangular().
• Reimplement DataGraph.square() by calling DataGraph.rectangular() with the same value for both dimensions.
• Update the relevant tests and documentation.