Refactor/samet/data

[samet-akcay]

Changes

• Apply pydocstyle to the changes.
• Refactor anomalib.dataset into anomalib.data
• Remove anomalib.data.anomaly_dataset
• Refactor anomalib.data.mvtec to use it as the default dataset

[ashwinvaidya17]

Looks good to me.

[djdameln]

Thanks. My only comment is about the download functionality. Now that we are using the MVTec class for other datasets that follow the MVTec directory structure, it might be better to separate the _download and _extract methods from the class. Users should be able to write their own implementations of these methods, so we could maybe add an interface (e.g. MVTecStyle(VisionDataset)) and n implementation specific for the actual MVTec dataset (e.g. MVTec(MVTecStyle)). But like I said in the comments we could do address this in a separate PR.