Add support to output a Report with a variable bias threshold

[neiljp]

Currently Report.__str__ just outputs fully-negative reports, and Report.to_dict gives all the data.

Ideally the output of the textual report would be tuneable. This cannot be easily set in the constructor, since this is called in each Detector.

Two options were discussed on slack:

• set a report attribute (or call a function) to specify the threshold, then use __str__;
• add an additional function (or maybe alternative to __str__) such as to_string, which accepts the threshold value.

I prefer the latter, but we can discuss further before implementing if necessary.