Implement classes facilitating a custom start.hh

I often feel restricted by the way our start.hh and the main.cc file is structured. The idea is to make it simple to run a problem in dumux so the start function is predefined. However it's currently annoying to implement your own main function since many of the steps in the standard start.hh method are just written out sequentially or in simple functions. It would be nice to make it more customizable by some OO programming. E.g. a simple class called the DumuxMessageGenerator and more important classes like a grid setup class or a parameter file / command line option reading class.

Simple examples would be:

1. I want to write a grid convergence test with a simple loop in main refining the grid and output the errors and rates.
2. I want to solve a problem with two grids.
3. I want to have objects or variables living outside the scope of a problem

These things are overly complicated now I think.