Convert DVector<DVector<T>> into a DMatrix<T>?

Question is self explanatory, but for XY problem purposes, my original problem is:

I have a `DVector`, and a function `Fn(T) -> DVector`, I want to apply the function to every element of the vector and get a `DMatrix` out where every column is the result of the function. Obviously I want this with as few copies/allocations as possible. The function constructs the column using `from_distributions` so I wonder if that can be used in place, so that no allocation has to be done.