Support integration over values of input variables in the partial dependency plots

[gkronber]

Currently, we need to set a value for each inputs and in each partial dependency plot we show a the dependence of the model when changing the corresponding input variable. It should be possible to show the distribution of model outputs also when we assume that selected input values are not known.

Suggestion for the GUI: allow three states for the checkbox list of inputs (show, hide, integrate).

Suggestion for the calculating implementation: instead of replicating the values for fixed variables sample values for the variables from the dataset. It is necessary to sample complete rows when multiple input variables are selected for integration.

[gkronber]

Prepared a preliminary implementation for this feature (see patch)