Give a better solution for `sum` of `Grad`s

[pvillacorta]

Now the sum of Grads is solved as follows:

+(x::Grad, y::Grad) = Grad(x.A .+ y.A, max(x.T, y.T), max(x.rise, y.rise), max(x.fall, y.fall), max(x.delay, y.delay))

But it would be nice if we could "stack" gradients:

Maybe this could be also extended to the rest of Sequence elements, so we could sum RF's, ADC's or even entire Sequence's in the same way.