Setting starting values for vector variable bridges

[blegat]

This is working for FlipSignBridge since it just multiply by -1 but for a linear map that is not diagonal, we cannot set the starting values variable by variable.

MathOptInterface.jl/src/Bridges/Variable/set_map.jl

Lines 182 to 192 in 5025b50

	function MOI.set(
	model::MOI.ModelLike,
	attr::MOI.VariablePrimalStart,
	bridge::SetMapBridge,
	value,
	i::MOI.Bridges.IndexInVector,
	)
	bridged_value = MOI.Bridges.inverse_map_function(typeof(bridge), value)
	MOI.set(model, attr, bridge.variables[i.value], bridged_value)
	return
	end

We would need to cache the vector of starting values in the bridge and then do the linear map in final_touch.

Alternatively, this caching could be done by BridgeOptimizer to simplify the job of each bridge but that caching isn't needed for FlipSignBridge so it could be a waste.