Make starting timestep more explicitly known

One-based or zero-based? Better be documented (in a way that doesn't
promote hard-coding in the clients!)

opm/simulators/timestepping/SimulatorTimer.cpp

@@ -30,7 +30,7 @@ namespace Opm
 
     /// Default constructor.
     SimulatorTimer::SimulatorTimer()
-        : current_step_(0),
+        : current_step_(FIRST_STEP),
           current_time_(0.0),
           start_date_(2012,1,1)    // A really arbitrary default starting value?!
     {

opm/simulators/timestepping/SimulatorTimer.hpp

@@ -50,6 +50,9 @@ namespace Opm
         /// Total number of steps.
         int numSteps() const;
 
+        /// First timestep returned from currentStepNum
+        static const int FIRST_STEP = 0;
+
         /// Current step number.
         int currentStepNum() const;