Support Conversion of Distribution Types

[akleeman]

It's often the case that we precompute a bunch of Distributions, but then need some subset of that for use as a function argument. For example we might have:

EvaluationMetric<MarginalDistribution> metric;
std::vector<RegressionDataset<X>> folds;
std::map<std::string, JointDistribution> predictions;

and we'd want to call cross_validated_scores(metric, folds, predictions), but since the metric takes a MarginalDistribution and our predictions are JointDistributions things don't work out gracefully.

The current approach is to specialize each function that needs to do this sort of conversion (see cross_validated_scores), but ideally we'd be able to set up a set of conversion methods that would let us do:

JointDistribution joint;
MarginalDistribution x = joint;