Measures inside derivatives are not expanded

When measures are expanded within an `InfiniteModel` via `expand_all_measures!`, any measure inside a derivative is skipped.
```julia
model = InfiniteModel()
@infinite_parameter(m, t in [0, 1])
@infinite_parameter(m, x in [0, 1])
@variable(m, y, Infinite(t, x))
@constraint(m, ∂(∫(y, t), x) == 0)
expand_all_measures!(m)
```
Hence, nothing happens with the above.