Add aggregate data class

[athowes]

In PR #390 we added a class for epidist_linelist data at the individual level.

We should also add class for aggregate linelist data.

It could be aggregated over delays:

Delay (days)	Count
1	20
2	35
3	50
4	40
5	60
...	...

Or aggregated over aspects of the window like:

Primary Event (date)	Secondary Event (date)	Count
2024-10-21	2024-10-23	15
2024-10-21	2024-10-25	20
2024-10-22	2024-10-24	25
2024-10-22	2024-10-26	30
2024-10-23	2024-10-25	35
2024-10-23	2024-10-27	40
...	...	...

Note that this type of data is appearing in our work on estimating right truncation delays. We also need models for this type of data soon.