Overloading constructors

[cscherrer]

I know this isn't part of this PR, but overloading constructors to construct something else can cause problems, the simplest being that something like StudentT(0, 1, 30) isa StudentT is false.

Originally posted by @sethaxen in #156 (comment)

My response:

Yeah, I agree that's a little weird. But the upside is pretty huge. This approach allows us to fine-tune code for Affine, and have that same code work for multivariate-anything. So implementing MvNormal gives us MvStudentT etc for free.

To me it seems worth it, but if you disagree we should talk through it

Let's discuss...