Questions about `getdof`

I have some questions about `getdof`. Here's the current docstring:
```julia
help?> MeasureBase.getdof
getdof(μ)

Returns the effective number of degrees of freedom of variates of measure μ.

The effective NDOF my differ from the length of the variates. For example, the effective NDOF for a
Dirichlet distribution with variates of length n is n - 1.

Also see check_dof.
```

First and easiest, is it right that we can now drop the "effective" from this? Or does it add anything?

Next, I'm wondering if we can pin down the behavior of this a little bit. Currently in MeasureBase I'm getting
```julia
julia> getdof(Normal())
NoDOF{LebesgueMeasure}
```
This is wrong, but I'm not sure of the best way to fix it. Is there a reason `getdof` can't just check the dimensionality of the root measure? [And, did we already discuss this?]

Then, I'm wondering what conditions should guarantee that `getdof` is defined? I don't think the root measure should be _requried_ to be `LebesgueMeasure() ^ n` for some `n`, since this doesn't hold for manifolds, including affine transforms.