improve figures in model notebooks

[petravanhoudt]

in the notebooks of the models, we've chosen a categorial plot and separated SNR level and whether or not a delay was included.
This improves the visibility of the different contributions. However, the number of figures has increased, which reduces the clarity of the notebooks themselves.

also the original reference values (combination Ktrans,ve etc) is not really clear from these figures.

Maybe we can think about a way to improve this in the future.