Enable output file aggregation over different time periods

The finalised CORDEX standard (https://zenodo.org/records/10961069) requires daily data files to span 5-year periods (e.g., 1981-1985, 1986-1990, 1991-1995, etc) and monthly data files to span 10-year periods (e.g., 1981-1990, 1991-2000, etc). If 5 or 10-year periods are not available the data can be less (e.g., 2011-2014 for historical daily and monthly data). It would be useful if this could be implemented into axiom.

Here is a screenshot of the example given by CORDEX for ERA5 data:

No files or tests are named. Start by locating axiom’s output-file aggregation entry point and compare its current period handling with the linked CORDEX standard. Done means daily outputs support five-year periods and monthly outputs support ten-year periods, while allowing shorter final periods when the full span is unavailable.

Written by the indexing model from the issue text.