added weighted mean calculation, fixes #176
added weighted mean calculation, fixes #176

cfpb/hmda-viz-prototype,awolfe76/hmda-viz-prototype,cfpb/hmda-viz-prototype,Kibrael/hmda-viz-prototype,Kibrael/hmda-viz-prototype,awolfe76/hmda-viz-prototype,Kibrael/hmda-viz-prototype