Add event aggregation and outputs.

[jetuk]

I am putting this here for anyone to have a look at it. It is still a WIP that I've been playing with for a while. I am not totally sure the interface between the aggregators and recorders/outputs is correct. However, it is functional.

Some remaining TODOs:

• Write some tests for event output.
• Implement event aggregation (probably assumes event duration is the value for now).
• Write some documentation in the book about how this all works.
• Implement to_dataframe() for the memory recorder
  • Maybe convert the memory internal data to polars dataframe(s) during finalise.
    • Would this make the aggregation functions easier?
    • Will probably consume more memory as a flat dataframe though.
  • Add Python interface to access this.
• Can aggregation functions be shared between parameters and outputs. Seems like we have duplicate implementations here.

[jetuk]

@Batch21 could you give this a high level review to see what you think?