Consider requiring two mutation method for all `DecisionNode`s

[arcondello]

We should consider requiring every decision to implement the following methods

class ArrayDecisionNode : public ArrayNode, public DecisionNode {
 public:
    // override the current state with a new one (specified as a flattened array).
    void assign(State& state, std::span<const double> values) const = 0;

    // override specific indices of the array
    void set_values(State& state, std::span<const std::tuple<ssize_t, double>> values) const = 0;
};

the responsibility would be on the caller to ensure the given state/changes result in a valid state for the decision, but each implementation should assert(...) for correctness.