Set result for a OneStepMethod

This is a very easy one...

I'm using the OneStepMethod for a computation which uses adaptive grid refinement between time steps. If the grid is adapted, all operators and thus the OneStepMethod have to be re-instantiated. I see that the original intention of the OneStepMethodResult was to store the overall time step information and display it after every successful time step. However, this behavior is broken when I build a new object. With my patch, a result can be cached using the result() function and recovered for the new object using the setResult()function.

The overall effect is that a new OneStepMethod object can be created without altering the displayed output.