Regression models support in fairness

[jakwisn]

It would be awesome to have support for regression models in dalex.fairness module. It should have some of the features pointed below:

• Density plot - showing the density of prediction for each subgroup
• PDP-like plot - for each subgroup
• Metrics that would allow for a plot like fairness_check
• Assuming that the fairness for regression models won't be as rich as for classification, there should be a way to transform the regression model/explainer into a classification model

[hbaniecki]

Let's actually introduce measures for regression models in a more 'paper/research' format (or find existing solutions).