new example: buoyant-bubble with a Boussinesq system (velocity extrapolation in time + pressure solver) (AGH eng. project by Michał Kowalczyk @Caastlefox)

new example: buoyant-bubble with a Boussinesq system (velocity extrapolation in time + pressure solver) (AGH eng. project by Michał Kowalczyk @Caastlefox) #412