Add difference equation/recurrence relation functionalities

[Dubrzr]

For example, it could be awesome if we could apply passband filters on generators such as:
y(t) = y(t-1) - 4y(t-2) + 2x(t+2) + x(t)

[Dubrzr]

Commit 792dfb6 implements difference equation that handles historical values. We also need to implement difference equation with future values. For example: y(t) = x(t-4) - 2*x(t+3)

[Dubrzr]

It would be awesome to compute the delay implied by the equation if possible.