the convergence behaviour can now be understood and the report step
information is printed, too. This does not affect `flow`, becase it
implements its own newton and time stepping routines.
the parameter is called `EclNewtonSumToleranceExponent`. if it is set
to 1, the specified tolerance will be used directly. (this is not
desireable in the general case though, because at the same result
quality, the sum error for large reservoirs can be larger than for
small ones.)
albeit, we scale the error only to the cube root of the pore
volume. the rationale is that the same amount of mass can get lost
"along" a line for each timestep.
maybe it would be a good idea to do something like this for time step
size as well because taking multiple small time steps currently allows
a much larger error in the result than doing it in one big step.
