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...
okay, I doubt that it will ever become a performance problem if the number of wells is not getting measured in thousands, but I guess adding everything into a std::map first and then using this to look up the index reads nicer as well. I'll change it tomorrow...