Natural aggregation normalizes values to the range mingrade -> maxgrade - it should use 0 -> maxgrade

While this behaviour is logical - it is different to the old behaviour - and since nothing in the spec says to change it - we need to update the code to work like "sum of grades" used to and write a behat test to lock it in.

Quick explanation of what should happen:
Manual grade item with a min grade of 10 and a max grade of 20
If I get 15 for this item - 20 should be added to the category grademax and 15 added to the total (Assuming automatic weights).

Manual grade item with a min grade of -10 and a max grade of 20
If I get 15 for this item - 20 should be added to the category grademax and 15 added to the total (Assuming automatic weights).

A Scale should use the values 0 - number items in the scale. The first item in the scale gets a grade of 1, 0 is for items set to "No grade"