Fix default workload for datasets with fewer than three columns

[ArijanaBohr]

The default workload in supporting_cliques and compiled_workload uses all three-way marginals. For datasets with fewer than three columns, this produces an empty workload and can cause mechanisms such as AIM and SWIFT to fail.

Datasets with fewer than three columns, however, are already supported elsewhere (e.g., MST), and compiled_workload is tested with a two-column domain when an explicit workload is provided. This leaves the default workload (workload=None) as the remaining edge case for datasets with fewer than three columns.

This PR sets the default workload degree to min(3, len(domain.attributes)), preserving the existing behavior for datasets with three or more columns while using the maximum possible degree for datasets with fewer columns. The supporting_cliques docstring is updated accordingly.

Testing

• Added regression tests for the two-column default-workload case.
• Ran the full test suite successfully.

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