When supplying values for \(\beta\) and \(\sigma ^2_{gm}\),
these parameters should be of a GM process and NOT of an AR1. That is,
do not supply AR1 parameters such as \(\phi\), \(\sigma^2\).
Internally, GM parameters are converted to AR1 using the `freq`
supplied when creating data objects (gts)
or specifying a `freq` parameter in simts or simts.imu.
The `freq` of a data object takes precedence over the `freq` set when modeling.