Merge:  Keep process ID during METS/MODS export (#172)

Extends the capabilities to copy data in Goobi as introduced in #170 and makes it possible to export the application’s internal process ID into a metadata element during METS/MODS export.

A second opearator was added:

       =    The value from the right hand side will be set on the left hand side.
            An existing value will be overwritten. If the right hand side doesn't
            exist, nothing happens.

Variable selectors may be used as right hand side (source) of an assignment. Examples:
 - `$process.id` − process ID
 - `$process.docket.file` − file name of docket
 - `$process.projekt.titel` − title of digitisation project

Data copier is called during DMS export and can be configured independently by config setting `copyData.onExport`.