Given a sequence of the DNA bases {A, C, G, T}, stored as a string, returns a conditional probability table in a data structure such that one base (b1) can be looked up, and then a second (62), to get the probability P(b2b1) of the second base occurring immediately after the first. (Assumes the length of seq is >= 3, and that the probability of any b1 and b2 which have never been seen together is 0. Ignores the probability that b1 will be followed by the end of the string.) Even though you don't need it, you may use the collections module if you want, but no other libraries