Data binning

[PiotMik]

Closes #20

Changes:

• [x] Data binning for continuous and discrete variables
• [x] Document structure refactoring

[PiotMik]

FYI @adamszczerba @AniaMatysek @joannakrezel

I binned the variables as discussed in the lectures. Please use Airlines_binned_train or AirlinesEncoded_binned_train for training your models (depending on your model, if it needs numerical or categorical data). Feel free to perform further transformations as per your model needs. I also updated the structure of the document to make a placeholder for the sections on models you have chosen.

Please git rebase your branch to include the newest changes (if needed) or open a new branch to work on your models.