MDL-46576 code_grade: Rename "Sum of grades" aggregation to "Natural"
MDL-46576 code_grade: Rename "Sum of grades" aggregation to "Natural" in sum of grades only settings
Merge branch 'natural_weights' of https://github.com/xow/moodle into natural_weights
MDL-46576 core_grade: removed grade/lib.php update_field()

(MDL-46967)
MDL-46576 core_grade: moved natural weight database changes into core
Merge branch 'natural_weights' of https://github.com/xow/moodle into natural_weights