Addendum to #402

Fixes bug with un-annotated heads in TreebankRetriever
Notifier user when it was impossible to parse SG
Implement aU.find for arrays
Implement aU.unique for arrays
Fix and refactor aU.unique()
More strict unique'ing in morph

Mild hack to avoid unnecessary dialect multiplication in greek. When
lemma and postag are the same, we show the form only once.
Show what's wrong when sg labels are incompatible
Even better error message in sg
Refactor error message
Further refactor of sg error message