Function plugins for gravity and boundary temperature.

These are plugins developed by Sanja Panovska at CIDER 2014. They describe the gravity and the boundary temperature as function parsers, which might be useful to a number of users. In this particular case the gravity function was used to prescribe the gravity profile on the moon, and the bottom boundary temperature was set to decrease over time. Maybe I can add an example test or cookbook later on.
Yes they are awkward. But they are used in some initial conditions and postprocessors. One could think about moving these parameters to the other plugins that want to use them, or provide an implementation in the interface, which throws an exception if it is ever called, but for now, I would leave it like this.