Added functionality to calculate free energy of a gaussian chain.

This treats the random walk as though it is made up of segments of a polymer.
The free energy is calculated based on the end to end distance of the first and
last points in the random walk. A probablity factor for transitioning into the
new state is calculated based on the difference between the free energy of the
current state and the new state. A random number is generated to see if the
new state will be accepted.

This is all very rough and more just to test out the idea. The physics is by
no way correct and the code is a bit messy as it stands. I will tidy everything
up so that the probablitiy evaluation is realistic.