Make Entity.layers a little more magical by checking if spans and box…

…es exist on both layers (#74)

* Make Entity.layers a little more magical by checking if spans and boxes exist

* add tests for new getattr() behavior

* increment

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Co-authored-by: kyleclo <[email protected]>

Merge pull request #43 from allenai/soldni/example

demo

fix fixture due to pinned pdfplumber