Consider dedicated archive function for extracting lags

[brookslogan]

For backfill-aware modeling and revision analysis, we often want to extract data for a time_value of ref_time_value - lag, as_of version ref_time_value, for all ref_time_value in requested ref_time_values and all lag in requested lags. We may also be interested in extracting data for a time_value of ref_time_value - time_lag, as_of ref_time_value - version_lag, for requested time_lag-version_lag pairs crossed with requested ref_time_values.

The first task is currently fairly easy with epix_slide, but:

• It may be non-obvious to users that this is possible. Having a nicely-named function may help discoverability of this functionality and improve code readability.
• If Benchmark epix_slide for lag extraction #374 finds this approach is too slow we will want a custom approach.

The second task doesn't as essential, but may be easy to implement if a custom approach is applied, and alternatives using multiple epix_slides would be even clunkier.