Update gpt.xml_snippet.xml

updated to better support pagination
Correctd error when harvesting CSW repository and one of the records has no DocID (harvester skips to the next record).
Several fixes to OAI-PMH harvesting protocol.
Federated search from Portal for ArcGIS to Geoportal Server.
Update README.md
Updated missing JavaDoc.
Minor warnings on harvestBody.jsp fixed.
Merge branch 'develop' into feat/Ags2Agp