Support for SNACS v2.6

[nschneid]

SNACS v2.6 English guidelines. Key changes:

• rename p.RateUnit -> p.SetIteration
• old p.Causer -> p.Force
• p.Causer now has a new interpretation
• p.Content added, but not used for English

Commit with changes to the STREUSLE code and data: nert-nlp/streusle@2786c8c

script for upgrading streusle.conllulex

[nitinvwaran]

Applied new labels and aligned the goeswith check with STREUSLE scripts in #5. The STREUSLE conllulex passes validation with the new changes.

[nitinvwaran]

#5 was merged to master, validator now supports SNACS v2.6 labels.