Improve decorrelation_time summation behavior

Getting a model with a long decorrelation time despite having a very short input data stream may cause the current `ar::decorrelation_time``to stop summing prematurely as the default`arsel`behavior is to integrate out no more than lag`N`given`N` data points.  Integrating out until the sum converges is dodgy just in case the sum doesn't converge.
