Follow-up tasks related to evaluation code

[laurejt]

On Spans:

• Refactor Span object so that we have separate classes for labeled and unlabeled spans
• Add set-like behavior to span object
• Add ordering method (using dataclasses)
• Refactor overlap_factor method

Evaluation Script:

• Refactor PageReferenceSpans and PageSystemSpans objects
• Have PageEvaluation use some graph structure instead of the ref_to_sys and sys_to_refs lists
• Review when spans should be allowed to be combined / split