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...
That sound good, a linear search is also acceptable for now, but I'd prefer to see std::find used rather than an explicit loop (that would be simpler to replace with a different search algorithm as well).