Created the CLI command bob bio pipelines transform

This MR creates the command bob bio pipelines transform that, well, transforms whatever you have in a database. This can be useful for:

• Crop all faces from a dataset
• Extract features using a particular feature extractor
• Preprocess some audio files using a particular pre-processor

This addresses and closes issue #175 (closed)

@mguenther, since you requested this feature, do you mind reviewing it?

Thanks