@Override
        protected Expression doConvert(Expression exp) {
            String indexName = ((IndexExpression)exp).getFieldName();
            if (attributeNames.contains(indexName)) {
                return null;
            }
            return new AttributeExpression(indexName);
        }
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