Changed Stanford CoreNLP to Stanza

[ReinierKoops]

This change should:

• Should speed up preprocess step
• Reduce the dependencies of the project (no more need for JDK and Stanford CoreNLP server installation)
• Stanza is native Python!

[ghost]

All CLA requirements met.

[ReinierKoops]

This change is proposed because I ran into a problem when trying to run this project ('preprocessing' step) on Google Colab. I had to run a separate StanfordCoreNLP server. With Stanza I can just run it without having to start a separate server instance for my Google Colab notebook (https://colab.research.google.com/drive/1QGBxUZb7_RlBjIJFC2O7XEovl80iQoB6?usp=sharing).