Use simple weighted mean of grades as default aggregation

Categories and totals had been computed based on (points earned)/(points possible) as is standard through 1.8 (and how teachers typically compute grades by hand). The 1.9 Beta 2 computed these values using "normalized mean of grades" which converts values to percentages before computing averages (non-standard). This meant that all the categories and course totals were wrong. Changing these to what is now called "Simple Weighted Mean of Grades" fixed this problem for most categories and the category sumaries in those cases were the same.