Deduplicate journal holdings in reporting

[phette23]

ACRL/IPEDS instructions have been to deduplicate journals held across multiple databases but we have had no feasible way of doing so in the past. The 360 Admin API should make it possible for us to track the journals as we go and deduplicate. We could store the journal ISSNs in a Set then count the set's members at the end. We would need to check that ISSNs are consistently available across database holdings. The holding object has an identifier and identifierType so there may be some complexity here, e.g. if the same publication has different identifier types across different database holdings.