WIP: Clean up track/trace

Name change: track and trace changed to previous and next. This will return one level.

Added track and trace to return a whole incoming or outgoing flow, which could include processes, economic events, and economic resources.

This relates to #93 (closed) which can be closed when this is merged, as soon as the question about limiting recursions is resolved.

NOTE: I did the method names, I could be missing some technical requirements. Also, please check the union I added for correctness and naming your prefer. In other words, @pospi and/or @mayel please take a close look and do whatever else needs to be done! Thanks!