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.
These parameters are awkward. Their values are pretty arbitrary, and most users would probably forget to set these parameters. I don't really have much of a better idea, though :-(