Wish list

## Measures from scratch
- [x] Specify equivalence class (wrt absolute continuity)
- [x] Density
- [ ] Probability measures
- [ ] Random sampling
- [x] Parameterized measures
- [ ] Measure of a set (for some extensible definition of "set")

## Composition of Measures
- [x] Superposition
- [x] Scaling
- [ ] Product measure
- [ ] Composition of parameterized measures ("mixture measures", formally a monadic bind)
- [ ] Reparameterization
- [ ] Push-forwards (or is it "pushes-forward"?)

## Extensibility
- [ ] Easily make measures from other packages a `Measure`

## Computations
- [ ] Check for absolute continuity
- [ ] Radon-Nikodym derivatives
- [ ] Lebesgue decomposition