Support dynamic multiplicity

[jqmp]

Currently the only way to create multiplicity is by setting fixed values with builder.assign or builder.set. This means the number of values can't depend on other values in the system. However, there are many cases where we'd like to generate a range of values dynamically.

For example, we might want to dynamically generate a parameter range:

@builder
@bn.output_sequence
def alpha(min_alpha, max_alpha, n_alpha_samples):
    return np.linspace(min_alpha, max_alpha, n_alpha_samples)

This will probably require some substantial architectural changes.