code: def new_bg(self,bg):
          
          counts = []
          for pos in self.logP:
              D = {}
              for L,lp in pos.items():
                  D[L] = math.pow(2.0,lp)
              counts.append(D)
          self.background = bg
          self.compute_from_counts(counts,0)
docs: m.new_bg(,bg)  -- Change the ACGT background frequencies to those in the supplied dictionary.
                            Recompute log-likelihood, etc. with new background.