Add average line functionality for multiBarChart and discreteBarChart

Hi,

This PR adds the averages functionality from the cumulativeLineChart to multiBarChart and discreteBarChart. 

It also extends the average functionality to enable average lines to be outside the range of the series, i.e. the average does not have to be the actual average, you can use it for anything, e.g a goal line to show the how something perform towards that goal. 

Test graph is added to the existing multiBar tests and two new tests are written for discreteBar that didnt have any tests.