BUG: correct damping with multiplicative trend
BUG: correct damping with multiplicative trend

Corrected the equations so that b**phi is calculated instead of b*phi
when using a multiplicative trend.

jseabold/statsmodels,josef-pkt/statsmodels,josef-pkt/statsmodels,josef-pkt/statsmodels,jseabold/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,jseabold/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,jseabold/statsmodels,statsmodels/statsmodels,jseabold/statsmodels,bashtage/statsmodels