Add summaries for trendbreaker outputs

[thibautjombart]

Both for the basic output, and the one of incidence2 objects. Ideally return a data.frame with:

• number of increases in training set
• number of decreases in training set
• number of outliers in training set
• number of increases in testing (recent) set
• number of decreases in testing (recent) set
• number of outliers in testing (recent) set