Disjoint decision variables not counted in ``decision_state_size()`` method

[JoelPasvolsky]

This surprised me:

>>> from dwave.optimization.model import Model
>>> model = Model()
>>> i = model.integer(5, lower_bound=1, upper_bound=10)
>>> model.decision_state_size()
40
>>> bins_set, bins_subsets = model.disjoint_bit_sets(20, 4)
>>> model.decision_state_size()
40
>>> disjoint_lists = model.disjoint_lists_symbol(100, 15)
>>> model.decision_state_size()
40

I found this confusing.

[arcondello]

Yeah, bug.

The DisjointLists variable doesn't have a state size, its successor DisjointList variables do. But when we calculate the .decision_state_size() we only look at the roots.