Measures inside derivatives are not expanded

[pulsipher]

When measures are expanded within an InfiniteModel via expand_all_measures!, any measure inside a derivative is skipped.

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.