Add convenience functions for plotting, extracting data

currently our functions return an `xarray.Dataset` which is quite nice if know how to handle it but pretty hard to extract what you want if you don't. So we need a couple of convenience functions to extract this, e.g.
`get_difference` (maybe this should even be a flag in our comparing functions) and also add some functions for plotting the data. Look at what shap provides here and make sure that these can be plotted easily