@Override
        protected void doVisit(Expression exp) {
            if (exp instanceof AttributeExpression) {
                attributeNames.add(((AttributeExpression)exp).getFieldName());
            }
        }
Replaces the 'index' statement of all numerical fields to 'attribute' because we no longer support numerical indexes.

@author baldersheim
config-model/src/main/java/com/yahoo/schema/processing/IntegerIndex2Attribute.java