SDD files load as plain logic circuits

[talf301]

If you load in a .sdd file currently using e.g. zoo_sdd, you get a plain logic circuit. This should probably return an SDD, or at least a structured logic circuit.
sdd = zoo_sdd("random.sdd")
Plain⋁Node(16953697995220010969)

[guyvdbroeck]

this is not a bug but a feature: if you also provide a vtree file path then it will parse as an SDD