Add functionality to retrieve decision boundary

[tlienart]

Use cases are typically visualisation but could also be further analysis like robustness (via margins) etc.

For some models they should be very easy to get (SVM, linear classifiers, LDA, ...) though sometimes dependent upon a threshold.

It's hard in general to describe the decision boundary but for the basic models we should provide a way.

[ablaom]

I suggest report as the interface point.

[tlienart]

yes that makes sense, so would you have a named field with a callable?

[ablaom]

Yes. The user does something like report(mach).get_boundary(threshold) or whatever.

In hindsight extras would have been a better name that report. I guess it's too late for that now.

[ablaom]

Or though we could switch and keep report for backwards compatibility.