Add shift operation for profiles to EDSL

[camargo]

In order to do rolling window constraints (i.e. no more than X minutes of this activity in any Y hour window) we need to be able to shift real profiles in time. The pattern will be:

let duration = Spans.ForEachActivity("my activity").accummulatedDuration();
return duration.plus(duration.shiftBy(X).negate()).lessThan(Y);

While we're at it, it can't hurt to allow shifting of discrete profiles too. It would be less used, but still could be helpful and not hard to implement.

[camargo]

@JoelCourtney Just noting for this one we should provide a couple examples in our docs with how to implement some "real world" constraints that require this operator