Fairness check

[jakwisn]

Architecture changed:

• fairness check now generates fairness object
• when plotted it is the same as for fairness check
• fairness check aggregates fairness objects and explainers
• label and cutoff work only for explainers
• cutoff is now a list of cutoffs for all explained models, so when new explainer is added it just appends to list. Same for labels
• polished other functions and plots
• examples now containing german credit data - smaller and faster
• vignette changed, now has more information about fairness_check
• better error handling