Standardising parts of the API.

Following a conversion in #2299, a number of instances where the API is inconsistent were highlighted. This issue is opened as a place to discuss and begin fixing these:

• ZeroChannelPaddedRecording saves it's parent recording to a parent_recording kwarg not a recording kwarg that seems to be used elsewhere.
• ZeroSilencePeriods stores the parent recording as a dict not a recording.
• CurationSorting uses a parent_sorting rather than sorting
• folder vs. output_folder argument.

More genreally, I think SI is becomming a very mature and widely used package and freezing the API / defaults as much as possible between versions would really enhance user experience / avoid bugs. I wonder if at some stage a full review of the API could be performed (this would entail multiple people going through every SI function / class and making notes on function names and default arguments inconsistencies or room for improvement). Then these could be discussed, frozen, and only changed in future with a very good reason. I wonder what people think about this.

Start by reviewing the API for ZeroChannelPaddedRecording, ZeroSilencePeriods, CurationSorting, and the folder/output_folder arguments, along with their current usages. Done would require an agreed, consistent naming and parent-object convention across these cases, plus a broader review and decision on which API and defaults should be frozen.