conano.py: added edge_type='spans' for edges from root to units
added get_text() function to retrieve text from a span node
exmaralda.py: removed acyclicity check (we only need acyclic coref chains)
exmaralda.py: add coreference chains as tiers
refactoring: renamed variables
exmaralda: added __span2event() which converts a list of node IDs into an event
exmaralda: ignore sentence annotations