AR(1) cumulation
[serieout]=autocum(seriein,rho)
* seriein = a (n x 1) or (1 x n) constant vector or a timeseries
* rho = a constant
* serieout = the vector (n x 1) or ts y such as:
* y(t) = rho*y(t-1) + x(t)
* y(t0) = x(t0) for the first observation t0