[maven-release-plugin] copy for tag dnet-openaireplus-mapping-utils-3.2.13
avoid NPEs
enabling to map structured objects as JSON strings
MapDocument implements a more general view of the pace model
cleanup
added custom conditions in the proto path navigation
merged branch ProtoMapping
removed protocolbuffers dependency from dnet-pace-core, Builders and Proto specific tests moved in dnet-openaireplus-mapping-utils, adapted dnet-mapreduce-jobs