Change `bind` to `compound`?

For push-forwards, we have
```julia
pushfwd(f, m)
```

If the `f` of a pushforward is noisy, it becomes a kernel, and the pushforward becomes a compound measure:
https://en.wikipedia.org/wiki/Compound_probability_distribution

This is currently called `bind`, because it matches the monadic bind operation from functional programming. But `bind` is unfamiliar to most users.

Maybe this should instead be
```julia
compound(k, m)
```
where `k` is a kernel. For functions, we'd have
```julia
compound(f::Function, m::AbstractMeasure) = compound(kernel(f), m)
```

For more discussion, see https://github.com/cscherrer/MeasureTheory.jl/pull/226