Timestepcontrol

This PR implements the adaptive time stepping for the Blackoil simulator. The TimeStepControl PR #662 for opm-core is need for this to work. 

Adaptive time stepping is enabled when the parameter file contains 
timestep.adaptive=true


It should say "number of linear iterations used", I initially assumed this was nonlinear iterations (until I read the code).

I think returning both would probably be a good idea, eventually (but this is ok for now), since that would be more stable considering that one may want to use various linear solvers with different characteristics.