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.

[Aarshpatel12]

Hi @petravanhoudt,

I would like to work on this. Since the current categorical plots are creating too much clutter, I plan to explore ways to consolidate these figures (perhaps using subplots or more compact visualization techniques) and ensure the reference values are clearly annotated.

I will start by analyzing the current plots in the model notebooks.