Show usage information from harvested metadata

[twagoo]

https://trac.clarin.eu/ticket/300 (also see #50)

In a similar way to #298, for each profile we could display usage information. The source of this information would be the [[CmdiDataSources]] harvested by the OAI harvester. A script would analyse these CMDI's and count profile usage (profile id from schema location or MDProfile element). For each profile the following numbers would be useful:

• Total number of CMDI instances based on it
• Percentage of CMDI instances that are based on it
• Number of CMDI instances based on it per collection
• Percentage of CMDI instances that are based on it per collection

This could be aggregated into an XML document or written directly to a (new) table in the Component Registry Postgres database.