Create example notebook with English Web Treebank data set

[frreiss]

Create an example notebook that shows analyzing the EWT data set

Major steps:

• Download the data set from https://github.com/UniversalDependencies/UD_English-EWT
• Read the dataset into DataFrames
• Write entire data set to a Feather file and read back in
• Display a parse tree
• Retokenize with a BERT subword tokenizer
• Show reconstructing a sentence's span using group by and aggregation
• Run document text through the Stanza EWT dependency parser (https://stanfordnlp.github.io/stanza/available_models.html) and compare the outputs against the gold standard. Or alternately use SpaCy's parser, with the caveat that it's trained on OntoNotes which has a slightly different schema.

[frreiss]

Closed by #198