This method is also based on stepwise extrapolation
defined by an initial stepsize z, however, one single
step usually consists of many substeps of the so called modified midpoint
method which are extrapolated to zero stepsize using a rational function
(Richardson extrapolation). Both the consecutive stepsizes and the number
of substeps are determined as functions of a predefined accuracy limit.
Its value was set to 10-6 m in the computations.