DICOM PDQ Service: Optionally perform the query on Study instead on Patient Level

[gunterze]

Optionally perform the query on Study instead on Patient Level using the Study Root Query/Retrieve Information Model - FIND SOP Class instead of the Patient Root Query/Retrieve Information Model - FIND SOP Class configurable by PDQ Service Descriptor attribute:

Name	Type	Description (LDAP Attribute)
Query Entity	string	Indicates if the C-FIND SCP is queried for a particular Patient or for Studies of a particular Patient. Only effective for DICOM PDQ Services (URI: pdq-dicom:{AETitle}). Enumerated values: Patient, Study. (dcmEntity)

If the query is performed on Study Level, use the Patient attributes of the returned Study record with the most recent Study Date (=> specify (0008,0020) Study Date as additional return key in the query identifier).

Improves #1631
Related #1711