multi-part figures often not merged

[cambro]

Much of the time, figures with multiple parts are segmented separately and not merged properly in post-processing, leaving multiple "figures" that cannot be matched to multiple "captions". This results in key figure parts being impossible to retrieve (i.e., they have no text and are associated with no captions). It's also likely that this is causing lower confidence on Figure proposals because "chopped up" figure and table elements are out of distribution (by definition rel. to our training data).

Revisiting the merging step for figures specifically is needed. Tables would also benefit from more sophisticated merging.

[cambro]

This is somewhat related to #103 and may be causing it in some cases.

[ankur-gos]

I'd like to revisit this and also come up with some test cases once we have the new coronavirus vertical live @cambro