Feature crossvalidation

[sfluegel05]

This includes 2 feature branches:

• old-chebi: Adds support for training sets based on chebi versions different from the test set, also adds support for setting a new chebi version altogether (including the test set)
• crossvalidation: Adds a new command that allows doing an inner cross-validation with holdout test set and k models trained on k train/val splits

I am not pulling this into dev yet as it requires restructuring the data dir