plot img data

[ValentinaGhidini]

I am not sure about the "df" in input of plot_img(): df should be the heatmap, i.e. a matrix, with the same dimension as the original image and values equal to the explanations for each pixel (in the correct positions!).

https://github.com/mfumagalli68/xi/blob/f27154c96f3911f5d1de8ac0e32b357c81707add/xi/plotting/plot.py#L36

Moreover, specify somewhere that we are using one-channel images (we could extend it to multi-channel images quite easily, but I need to think how to represent the final heatmap).