Function File: avgbits = laverage (codebook, problist)

Compute the average word length SUM(i = 1:N)* Li * Pi where codebook is a struct of strings, where each string represents the codeword. Problist is the probability values. For example

               x = {"0","111","1110"}; p=[0.1 0.5 0.4];
               laverage(x, p) ⇒ ans = 3.2000