AnalyzedSeries __compute_change_points() and __group_change_points_per_time() could be @properties instead

[henrikingo]

https://github.com/apache/otava/blob/8bb5e8eaaf8d0880650ad382c557963912e9454d/otava/series.py#L235C13-L235C15

I feel like it would be more natural python to have self.change_points, self.change_points_by_time be @properties. It would also save some computation in cases where the latter is never used, in which case it would never need to be computed.