Add `filter` for `epi_archive`

[brookslogan]

[brookslogan]

This is one of the missing operations for archives that makes it awkward to debug and work with. Often I want to filter to a one or two geos for initial experimentation or for debugging. Currently, I can do that, but it involves working with the $DT directly; this is inconvenient as a user, and it also somewhat stands in the way of encapsulating away $DT and only providing access via (more expensive) methods that will clone it to prevent memory model violations.