Update plots

[radka-j]

This PR:

1. adds option to plot features vs outputs using filled error regions rather than error bars (which are still the default)
   • this was the original version of the plot which we removed as we thought it was misleading but it would be nice to have the option to make this plot in some use cases (I have seen it elsewhere)
   • enable by setting error_style=fill

2. adds plot of true vs predicted values for each output (not by feature as the plot above)

3. For both plots the user has the option to pass in names of the input/output features for labelling. This is now demonstrated in the quickstart notebook

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[sgreenbury]

Looks great, thanks @radka-j! I just left some small suggestions below.