Consistent API for DiscreteDistribution and subclasses (like DiscreteDistributionLabeled)

[sbenthall]

DiscreteDistribution has two public attributes, pmv and atoms.

DiscreteDistributionLabeled subclasses DiscreteDistribution, and has a pmv attribute. But instead of atoms it has data, though these attributes have identical comments/documentation.

        Discrete point values for each probability mass.
        For multivariate distributions, the last dimension of atoms must index
        "atom" or the random realization. For instance, if atoms.shape == (2,6,4),
        the random variable has 4 possible realizations and each of them has shape (2,6).

Is there a good reason for this change? This makes it harder to swap a labeled disitribution in for an unlabeled one.