BUG: fix indexing of seasonality when using amplitude
BUG: fix indexing of seasonality when using amplitude

ceholden/yatsm,jmorton/yatsm,valpasq/yatsm,c11/yatsm,c11/yatsm,ceholden/yatsm,jmorton/yatsm,jmorton/yatsm,valpasq/yatsm