need a way to loop over quadrature points

@hammy4815 and I were looking for a lower-level abstraction that gives us access to the quadrature points used when assembling a matrix. (This is for topology optimization where the degrees of freedom are specified on some external grid, not on the FE mesh.)

I think all we need is a API like the following, given a quadratic form `a` and two FEFunctions `u,v` on some mesh `Ω`:
```jl
a(u, v) = ...some quadratic form...
quadratureloop(a, u, v, Ω) do aₓ, x, wₓ
... do something with aₓ(u, v) at the quadrature point x with quadrature weight wₓ ...
end
```
so that, for example, `∫a(u,v)dΩ` could be computed with:
```
integral = 0.0
quadratureloop(a, u, v, Ω) do aₓ, x, wₓ
integral += aₓ * wₓ
end
```
(In our case we want to do something different with the quadrature points and weights, not just an integral)