Introduce even / odd event splitting.

This PR adds an improved event splitting for 2-fold validation. New categories are introduced, postfixed _even and _odd, which contain the event number based splitting in the category selection statements. The preprocessing, training and plotting pipelines are adjusted accordingly. The default categories are now baseline_even for plotting etc (--category-name), and baseline_odd for the training (--training-category-name).

When merged, this closes #7 (closed).