Compute the exponential weighted moving average of a series of values. The time at which you insert the value into `Ewma` is used to compute a weight (recent points are weighted higher). The parameter for defining the convergence speed (like most decay process) is the half-life.

e.g. with a half-life of 10 unit, if you insert 100 at t=0 and 200 at t=10 the ewma will be equal to (200 - 100)/2 = 150 (half of the distance between the new and the old value)