Switch to CoNLL-U format

The most visible change is that from version 0.3.0 onwards, sticker2 uses the CoNLL-U format. Besides that there were many other improvements:

• Switch from CoNLL-X to CoNLL-U as the file format.
• Much-improved error messages.
• Add TdzLemmaEncoder This encoder uses the edit tree encoder, but performs the necessary pre- and postprocessing to produce TüBa-D/Z style lemmas.
• Add an option to ℓ2-normalize sinusoidal embeddings and make it the default. This improves model convergence (suggested by @twuebi).
• Support encoding of the full features column as a string (rather than individual attributes/values).
• Permit setting a default value for features. This is useful for using features that are not annotated on every token.
• Add the filter-len subcommand. This filters a corpus by the sentence length in word or sentence pieces.
• Improvements to serialization of encoders: remove phantom data and storing the feature <-> number bijection twice.
• Update to libtorch 1.5.0.

Models trained with versions prior to 0.3.0 are not compatible with this version. At the moment we only provide compatibility of models with each version y in 0.y.z.