Units: add 'Timestep' dimension

the motivation for this is the 'TSTEP' eclipse keyword.

the reason why this dimension is not simply called 'Time' is that in
eclipse, different keywords might use different units, e.g. one
keyword could use seconds, another could use years while a third uses
days. As an added bonus, the used time units may be different for
different scales but identical in others (e.g., for one keyword times
might be specified in 'days' for the metric as well as for the
labscale, but in 'days' and 'hours' for another second keyword.)

opm/parser/eclipse/Units/UnitSystem.cpp

@@ -145,6 +145,7 @@ namespace Opm {
         system->addDimension("FlowVolume", Metric::FlowVolume );
         system->addDimension("Rho"       , Metric::Density );
         system->addDimension("mu"        , Metric::Viscosity);
+        system->addDimension("Timestep"  , Metric::Timestep);
         return system;
     }
 
@@ -163,6 +164,7 @@ namespace Opm {
         system->addDimension("FlowVolume"    , Field::FlowVolume );
         system->addDimension("Rho"  , Field::Density );
         system->addDimension("mu"   , Field::Viscosity);
+        system->addDimension("Timestep", Field::Timestep);
         return system;
     }