EclProblem: clean up the time management code

- when an episode/report step is over, the next is started by endEpisode()
- the problem does not deal with updating the simulation time anymore
- rename `episodeIdx` in to `reportStepIdx` the 'EclWriter' because
  this variable is -- and always has been -- the report step number
  used by some parts of `opm-output`'s ECL writing code (the report
  step number is equivalent to the episode index plus 1). IMO, the
  output and parser code should be made more consistent in regard of
  whether it expects 0-based or 1-based indices, but this is a story
  for another day.

tests/test_ecl_output.cc

@@ -131,6 +131,7 @@ void test_summary()
     simulator->model().applyInitialSolution();
     Opm::data::Wells dw;
     bool substep = false;
+    simulator->startNextEpisode(0.0, 1e30);
     simulator->setEpisodeIndex(0);
     eclWriter->writeOutput(substep);
     simulator->setEpisodeIndex(1);