Altered the step length in the take_step routine.

In order for the rough free energy evaluation to be valid each step should be the same
length. That is to say that in the model of a Gaussian chain made up of N segments each
of length a the rms end to end length is

        r = sqrt(N)*a

Each step taken should be equal to length a in order that the form of the free energy
used is valid

        F = 3/2 * KbT * r^2 / Na^2