Relocate SizeClassifier to later stage: yields better results for reference handling

Pending testing on corpus; this seems not to have broken any regression tests. Will ensure that this is the case.
Revert SizeClassifier position: breakage on Zotero/Mendeley docs
Commit comment on design decision re. compensation for user error
Add test document for mis-placed heading halfway through reference list
Initial implementation of multiblock reference parsing
Regression test for multiblock reference parsing
Adjust multiblock reference parsing to join line-split references to correct sibling
Initial refactor of multiblock reference parser to arbitrary depth
Split line in reference parsing
More rigorous control flow including null reference checks
Include fail count when spanning multiple sections in multiblock reference parser
Remove unnecessarily verbose debug message
Test document for abitrary depth of multiblock reference headings
Extra debug statements for when references are left in situ
Add test for multiblock parsing across an arbitrary number of sections
Add document for trailing acknowledgement (endgame) test in reference parser
Test for reference block with trailing acknowledgement
Test document for fail condition when consecutive non-ref blocks added after multi-block break point
Fix bug in multiblock handler referencing incorrect item
Change reference to "sibling" to "item"