Add option to pass in function for replaceing p_LV and p_RV

[finsberg]

Here we add the option to pass a function the computed both the LV and RV pressure for a given volume.
The reason for doing it this way, and not supplying one function for each chamber is to make it possible to pass in a function that solves a finite element model and not having to solve the model twice (one for the new LV volume and one for the new RV volume).

In the future we should probably think about if there is better way to do this.