Allow multiple parent steps
A step can now have multiple parents, ie multiple steps can trigger it. Note: the ProcessingStep API has changed: parent is removed (since it's not used in the code) and requires is now a list of strings.

Related to https://github.com/huggingface/datasets-server/issues/1014
Allow multiple parent steps: A step can now have multiple parents, ie multiple steps can trigger it. Note: the ProcessingStep API has changed: parent is removed (since it's not used in the code) and requires is now a list of strings.

Related to https://github.com/huggingface/datasets-server/issues/1014