improve incremental tree reading &c.

- treebank: option to add root node to trees
- incremental tree reading: require starting brackets to be on new line
  only preceded by whitespace. this avoids treating e.g. $( in export
  files as start of a bracket tree.
- BracketCorpusReader and incremental tree reader:
  handle frontier (empty) non-terminals
- treedraw: no longer special case preterminals
- treedraw CLI: --fmt=auto to use incremental reader with multiple files
treedraw: use center of gravity of leaves instead of children