Fix SimulatorTimer-related bug in EclipseWriter.

Since writeTimeStep() now 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.
Merge pull request #485 from atgeirr/fix-initial-call-eclipsewriter

Fix SimulatorTimer-related bug in EclipseWriter.