Generate all valid models for a given logical formula and/or CNF

[mattxwang]

Motivated from a conversation with Lisa: on top of model counting with .wmc(), it would be moderately useful to list out all valid models that satisfy a logical formula, and (optionally) the associated weights with each model.

Since this is computationally hard, we should do this on-demand only from the user. We can probably use the existing PartialModel struct; we could then return a tuple for each model, which contains both the assignments for each variable, and the associated weight.

I'm thinking something that looks like this:

let weights = WmcParams::new(/* ... */);
let formula: DDNNFPtr = builder.compile_cnf(); // can be whatever approach we'd prefer
let valid_models: ValidModels = formula.enumerate_models(&order, &weights);
for model in valid_models {
  println!("weight: {}, assignments: {}", model.weight(), model.assignments())
}