Implement new "Natural weighting" interface

This feature is an improved way of setting and displaying weights for grades in a category.

It is described in detail in the specification: http://docs.moodle.org/dev/Natural_weighting

This aggregation method will be able to combine many of the current aggregation methods into one. If that is so then we can potentially remove quite a lot of code from the gradebook, improving speed and readability.

However, to be safe, we plan to implement this in stages:

STAGE 1) Implement natural weighting as part of the Sum of Grades aggregation type, with admin settings that allow them to disable all other aggregations if they choose. (Moodle 2.8)

STAGE 2) Once people have been using this for a while, explore the possibility of removing other aggregation code or making them into optional plugins. (Moodle 3.0 perhaps)