Feeding multi segment recording to sorters

[khl02007]

@samuelgarcia @alejoe91

https://github.com/SpikeInterface/spikeinterface/pull/489

By the way, I think the users should always give to sorters an AppendRecording and get back a SplitSorting. For the sorters that only operate on ConcatRecording (and not on AppendRecording) they should just convert the AppendRecording to ConcatRecording under the hood. This is because AppendRecording/SplitSorting is obviously the correct way to handle multiple segments. Maybe can print a message to let the user know that this is happening. Thoughts? @alejoe91 @samuelgarcia