Add function(s) to process _RELS_EXT.rdf

[Tyrica Terry Kapral]

This is a bit complicated because, ideally, 'collection_id' would be in all digital item-level base layer CSV, rather than a separate CSV.

In data_layer_config.py:

DIGITAL_ITEM_RDF_MAP = {
      'collection_id': {'bs_exp':['Description > isMemberOfCollection[rdf:resource]',]}
}

For example of RELS_EXT file, see pitt_31735047439256_RELS-EXT.rdf.

[Tyrica Terry Kapral]

Modify if statement block to include 'collection_id' in extract_base_layer.py:

if u == 'copyright_status':
                # looks for attribute value and handles exception if no attribute
                results = bs_object.select(exp)
                try:
                    field_data += results[0]['copyright.status']
                except:
                    field_data += ''