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.
Maybe add that the formula is stated in section 'Gravity model/Function' and that the function can be an expression in terms of the variables 'xyz' (I assume)?