Update qa_srl2018.py

Support 'domains' kwarg in loading

fix data bug - empty prep slot; replacing with underscore

Include dataset versions - support both "v2" and "v2_1" (default)

improving 'verb_form' to be the 'stem' form of the verb

adding 'is_verbal' and 'verb_form' fixed columns, to align with qanom and qa_srl2020

initial commit