merging trunk changes with IIS-CDH-5.3.0 branch
creating IIS-CDH-5.3.0 branch
#1022 removing ExtractedDocumentMetadata envelope: origin info is not required
#1022 introducing ExtractedDocumentMetadata envelope required for collapsing PMC metadata records
removing unused import statement
fixing indent
#1017 introducing PMC extracted metadata schema
introducing PmidMapping schema
Adding info on where to find types of citations produced by PMC citation ingestmodule
rename a field