Enable PyPOTS to work on the anomaly/outlier detection task

[WenjieDu]

1. Feature description

Add a subpackage in PyPOTS named anomaly_detection to include models for the anomaly/outlier detection task.

2. Motivation

Anomaly detection is a prevalent task in time series analysis, which PyPOTS should support.

3. Your contribution

Will make a PR to initialize this subpackage and implement models in it for the anomaly detection task.