I am not so sure that they should be in the StructuredOutputMachine anyway. See for instance the Machine,
LinearMachine, and KernelMachine case. The LinearMachine has a pointer to the features, but that is not
the case of the KernelMachine, which gets the features through pointer to the kernel (i.e. the kernel has pointers
to the features). Therefore, I think it makes sense that the StructuredOutputMachine does not contain the features.