EclipseWriter: attempt to fix the well summary output for activeWells != totalWells

The summary files now always features all wells which ever appear in
the deck in every timestep (even if they are not specified for the
time step). This _should_ fix the crashes when writing Eclipse output
in the Norne deck, but I have no idea if the resulting summary file is
correct. More testing would be thus highly appreciated...
The getWells() method is linear in the number of wells, so this becomes quadratic, in every timestep. Could you move the getWells() call somewhere up the call stack so that we do not have this? It is not a very big performance problem with SPE9 and the current code, but as we refine and optimise the simulator it could become a minor bottleneck.

If this is very hard to do, I can accept the current solution.