More support for Multivariate distributions

[vinayakpathak]

It might be nice to be able to do the following kinds of things with multivariate distributions:

1. Marginalize over a subset of variables.
2. Calculate conditional distributions conditioned on a subset.
3. Assign names of arbitrary types to these variables instead of non-negative integers. For example, might be nice to be able to do something like

val g = MultivariateGaussian(...)
val mg = g.marginalized(List("A", "D", "E"))

This should give us g marginalized over the variables named A, D, and E.