thetaramptime: $\tau_\theta >0$

This is used to damp out transient barotropic motions by initializing the value of $\theta$ in the free-surface solver to 1 and ramping it down to the value of $\theta$ specified below. The $\tau_\theta$ parameter determines the timescale, in seconds, over which this is damped, such that, if the value of $\theta$ specified below is given by $\theta_0$, then the value of $\theta$ that is used in the calculations is given by

$$\theta(t) = \theta_0 + (1-\theta_0)\exp\left(-\frac{t}{\tau_\theta}\right) .$$