timestepper classes

[jshipton]

At the moment we have 2 time stepper classes: one does the usual nonlinear iterations (for solving shallow water equations or compressible Euler or whatever) and one is just for advection. This could do with a bit of refactoring as it is not sustainable to add diffusion, or physics, separately to each class. This could be improved by adding to the base class to that it handles the diffusion/physics but I think a more substantial refactoring could help with other timestepping methods (i.e. the exponential integrators and frequency averaging schemes).