Problem: DNA-2, CG counting

Problem Statement

Given a strand of DNA determine the percentage of cytosine and guanine nucleotides present in the genome. The strand is represented by a string containing only the lowercase characters 'c', 'g', 't', and 'a'. Your method should return the percentage (between 0.0 and 1.0) of 'c' (cytosine) and 'g' (guanine) in the genome. For example, if half of the nucleotides are 'c' or 'g' your method should return 0.5.