Provide wrapper which connect simulator and writer

This wrapper class aggregate all the result objects (state, timer
etc.) and register itself for notification with the simulator. At
every timestep the simulator should provide a notification. The
wrapper will receive this and pass all the state objects to the
writer(s) which creates a new timestep.

Currently this doesn't work out-of-the-box because only the
two-phase simulator sends notifications and the writer only
understand the state of blackoil simulators, but these two concepts
should be unified.
Move shared object hack into more useful generic

The template function share_obj let you pass regular references
as shared pointers, which makes it easy (perhaps too easy) to quick-
fix old code which pass references and have stern warnings about the
lifetime rules of the objects in the documentation section instead.