modgrade form element has messed up value processing

This was first detected when testing MDL-47684 where validation was introduced into moodleform_mod and the value for a modgrade form element was processed by updateValue and came from _submitValues rather than _defaultValues.

The problem comes because:

• In _defaultValues an int is used and is mapped to either a scale (negative value) or point (positive value)
• In _submitValues as the form element was an array of elements the value for a submit form is an array.

I suspect the design of the modgrade component here is intrinsically wrong in that the modgrade element is based on a group and has no actual value.
In the event a form is submitted the updateValue event is called for each individual element in the modgrade group as well as the modgrade (group) element itself.
A bit of debugging shows that the updateValue is called first for the modgrade element, and then for the individual elements that make it up eg:

1. scale
2. scale[modgrade_type]
3. scale[modgrade_scale]

I would suspect that because of this ordering the setting of values logic in modgrade would be undone in the situation of a form submission.
It is also the root of the problem detected in MDL-47684