preferences: fixed get_chain_length()
- input will always be a known entity, so we don't need
  an else case
- (the else case would have caused an error when calling math.log(0))
_resolve_pronominal_anaphora(): fixed call to filters.distance()
preferences: fixed get_chain_length() generator processing
main: reincluded pronominal handling. END OF EMERGENCY
fixed _resolve_pronominal_anaphora()
- so far, unknown anaphora weren't registered in the mentions dict
minor: removed print/debug code from resolve_anaphora()
main: reimplemented output_with_brackets()