tweak head finding

- PTB headrules: add rules for NP
  these are not part of Collins' rule list because they are
  special-cased with separate rules; since we do not incorporate these,
  we include heuristic rules instead.
- Negra headrules:
  - fix spelling of '-' label to '--'
  - add rule for DL label
- head finding: when no matching head from rules is found, use its
  direction to select a default head.