Sandwiching should accept a sub optimizer

[odow]

This is not correct because F() may not exist:

MultiObjectiveAlgorithms.jl/ext/MultiObjectiveAlgorithmsPolyhedraExt.jl

Lines 48 to 49 in 214db33

	optimizer = typeof(model.inner.optimizer)
	δ_OPS_optimizer = optimizer()

I'll think about how we should fix this.