export: filter or group by operational period

[caver456]

grouping: each operational period could be its own top-level checkbox group, with that OP's owned assignments as checkboxes within that group.

filtering: a cyclic field to choose which OP's assignments to show would reduce the length of the checkbox form. On occasion, there may be need to show multiple OP's assignments in the export form; this could either be done with a multi-select cyclic field, or with a cyclic option 'all'.

Collapsible checkbox groups (one group per OP), instead of filtering, may accomplish both goals.