attrib only collect supported fields from ABOUT files

[chinyeungli]

All the non-supported fields in the ABOUT files will not be collected in the attrib.

For instance,
Users can generate ABOUT files with custom field such as internal notes if the input has this key and user properly state in the MAPPING.CONFIG.
However, when doing the attrib, the tool will create ABOUT object from ABOUT files which only contains all the supported fields, all the non-supported fields will not be collected and therefore cannot be used in the attrib. In another word, users will not be able to put internal notes in the attribution.