Segment seasonality offset #805

[VargBurz]

Closes #805

Problem:

Seasonal segment was shifted to the left by one index

Root cause:

Wrong seasonality_offset calculation algorithm that discards fractional part of number.

Changes:

• replace floor division with math.ceil in seasonality_offset
• move duplicated code from AnomalyDetector methods detect and process_data to new method get_seasonality_offset
• add get_seasonality_offset unit test
• anomaly detector: add to: segment['to'] field to the cache after train
• use new AnomalyDetectorSegment class instead of dict in anomaly detector
• new detections_generator method which checks if values of data are out of bounds

Before:

After: