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.
OK. Maybe add a sentence of the following kind to the general description of the model at the bottom of the file:

"Models that describe boundary temperatures internally need to implement functions that provide the temperature at individual points, but also functions that return the maximal and minimal values of the boundary temperature. For most models, this is no problem as they know what the temperature is everywhere, but the current model only gets a function that can be evaluated at individual points but for which it is not possible to determine the maximal or minimal value with reasonable computational effort. Consequently, when using this model, you need to not only set the function object, but also provide values for the `Minimal temperature' and `Maximal temperature' parameters in the input file."