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...
Merge pull request #625 from andlaus/fix_wells_in_summary_output

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