Calculates the Shannon entropy for a Crosswalk
xwalk_entropy.RdThe more potential codes that a crosswalk will allow an intial code to become, the higher the entropy. The entropy (S) is given by $$S = -\Sigma \Sigma p log p$$ where p = 1/n and n is the number of potential codes. a single code can map to, natural logs are used in the calculation. The inner summation can be is the sum of n iteration of 1/n, so the equation can be simplified to $$S = -\Sigma log p$$