Add feature to recombine a metadatum out of several others

Adds a feature to recombine a metadatum out of several other metadata elements in other hierarchical levels, meanwhile adding formatting. This can be used to create a metadatum DatePublished on each issue during newspaper export, combining into it the metadata values from the predecessor structure levels year, month and day.

Required configuration:

    copyData.onExport=/PublicationYear[0]/PublicationMonth[*]/PublicationDay[*]/Issue@DatePublished \=format "%1$04d-%2$02d-%3$02d" #1@TitleDocMain #2@TitleDocMainShort #3@TitleDocMainShort

**Prerequisite:** Development of this feature needed to be based on #170 *Keep process ID during METS/MODS export*