Introduce a structure that contains signals that point to functions
that do the actual assembly loop. The setup of scratch objects etc
continues to happen at an outer level, but on every cell we then
simply call each slot in the signal, and thereby add up terms as
appropriate for an equation or approximation.

This requires us to store a list of such objects somewhere. But it also allows us to
remove the SimulatorAccess object from the argument list of these functions.

… into a slot.

This requires making the FreeSurfaceHandler public in the Simulator class. If merged,
I will move this class into its own header file at a later time.