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, the PR has been updated. On closer inspection, I noticed that this is _not_ quadratic but it is in O(m*n) over the whole of the simulation, where 'm' is the number of report steps and 'n' is the number of wells in the deck. Thus the std::map approach is actually a bit slower (it goes to O(m*n*log(n)), but it looks nicer IMO...