Add getter and setter for the CO array under PDAcroForm

The CO-array is a part of the AcroForm object, which defines the order in which fields are calculated (see Table 218 in the PDF ISO specs). It contains (indirect) references to the calculated fields.

It would be nice to have a direct getter and setter for it under PDAcroForm. Also, if the references could be automatically converted to point to the actual PDField objects, that would be great.

In order to do that, the values from this array need to be compared to those returned by the getCOSObject method of the PDField objects returned by the getFields methods of PDAcroForm.