`productmeasure(::Vector{<:Dirac})`

Currently, a product of `Dirac`s works like this:
```julia
julia> productmeasure(Dirac.([1,2,3]))
ProductMeasure([Dirac(1), Dirac(2), Dirac(3)])
```
In some cases, we might prefer to write this as
```julia
Dirac([1, 2, 3])
```

This is much simpler, at the cost of an allocation.

For cases where we end up building an array of `Dirac`s only to later take a product, we should look for ways to think ahead, so we can jump straight to this second representation. In this case we can often avoid the allocation.

I don't think there's a need to have a single canonical form for this. We can instead add some methods to deal with `ProductMeasure{<:AbstractArray{<:Dirac}}` and take advantage of this structure.