Make Natural aggregation the (single) default aggregation method

Currently there are a number of aggregation methods. The new "Natural" aggregation method can cover most of the functionality of all existing aggregation methods.

On new installs, this should be set as the default aggregation method.

It is also possible to hide the other aggregation methods by setting it as the single selected aggregation method at the site level.

Settings for both are defined at Site admin > Grades > Grade category settings.

If there is only one aggregation method available, then aggregation controls should be hidden in the gradebook setup pages and wherever aggregations are named.