Constructs a RandomGridIterator that iterates pseudo-randomly over the
integer grid [xorigin, xorigin+width-1] x [yorigin, yorigin+height-1]
using the random sequence generator 'random'.
Given a value of X and dX/ds at s, a stepsize h, an allowed error, and a way to
compute dX/ds at new points, this method will return a new value
of the dependent variables X at s, where s incremented over s by an amount
not more than h, but perhaps much less than h, but with an error in X of less
than the desired error.