Fix SimulatorTimer-related bug in EclipseWriter.

Since writeTimeStep() now (after a recent PR in opm-autodiff) gets called also before the first timestep is done, the EclipseWellTotal::update() method was in error, always calling SimulatorTimer::stepLengthTaken(). That method cannot be called if the
timer's currentStepNum() is 0, so that is checked for and intercepted.

The total rate in that case is of course zero.