Added new grouped bar chart class.  This class needs work to better generalize.  Right now, much of the code is specific for showing a mutation spectrum, which is a bit different that a standard grouped bar chart.  For a mutation spectrum the bars in each group will be the same number (3), but the domain of the x will vary.  For example, the 3 bars for the category A (nucleotide) will consist of the 3 other bases (G, C, and T), but for the category C, it will consist of the 3 other bases (A, C, T).