use `std::vector::emplace_back()` in `DistVariation`

See !150 (comment 8081487)

changed milestone to %Alma9 migration

added To Do label

added to epic &3

mentioned in merge request !150 (merged)

assigned to @paconnor

removed To Do label

added Ongoing label

mentioned in merge request !165 (closed)

mentioned in commit 4088b5f7

mentioned in commit 95f5a2ed

mentioned in merge request !166 (merged)

closed