proiel/proiel-treebank: Release 20180408

This release adds two new texts: Cicero's De officiis and Palladius' Opus agriculturae, as well as additions to the Greek New Testament, Jerome's Vulgate, Cicero's Epistulae ad Atticum and Herodotes' Histories.

There are corrections to morphology, lemmas and dependencies across the collection. A number of invalid antecedent IDs have also been corrected.

Text divisions now have their own IDs and aligned texts have the IDs of aligned text divisions indicated.

The documentation has been updated to include a breakdown of sections that have not yet been annotated.