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.
Ah, but I will change the minimal value to 273 K, which is consistent with unit of the maximal value.