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


Thank you for adapting (ha ha) to use the existing hierarchy option, there is just one fly in the ointment: there is no default here, meaning that an exception will be thrown if the ParameterGroup does not contain the group "timestep". Clearly this is a little less than user friendly, but there is one way to do this, using the getDefault() method. That would be:
```
const parameter::ParameterGroup& timeStepParam = param_.getDefault( "timestep", param_);
```
which would return the main ParameterGroup if no subgroup "timestep" was found. This means that simply saying "adaptive=true" would be enough if there are no conflicts, but that the nested "timestep/adaptive=true" can be used instead if we want "foobar/adaptive" to control something different. This is most likely for words like 'tolerance' etc.

Since the above getDefault() may look a little strange at first, perhaps we should make it a separate function getGroupDefaultToParent(const std::string&) or similar, in the ParameterGroup class.