DS-1933 Enhancement for cli batch import regarding collections

Jira Issue: https://jira.duraspace.org/browse/DS-1933

Documentation (to be included in https://wiki.duraspace.org/display/DSDOC5x/Importing+and+Exporting+Items+via+Simple+Archive+Format)

* Importing (per item) into multiple collections from the command line administration tool

  If you omit the -c flag, which normally is mandatory, the
  ItemImporter searches for a file named <collections> in each item
  directory. This file should contain a list of collections, one per
  line, specified either by their handle, or by their internal db
  id. The ItemImporter then will put the item in each of the specified
  collections. The owning collection is the collection specified in
  the first line of the collections file.

  If both the -c flag is specified and the collections file exists in
  the item directory, the ItemImporter will ignore the collections
  file and will put the item in the collection specified on the
  command line.

  Since the collections file can differ between item directories, you
  have more fine grained control of the process of batch adding items
  to collections.

* Importing with BTE 
   
   The DSpaceOutputGenerator, that writes the metadata in DSpace
   Simple Archive Format, has been updated to produce the collections
   file, if a metadata field named collections (reserved word) exists
   in the original metadata. This is mainly applicable to the CSV
   input format which is more flexible, but could also be
   implemented with a Modifier that adds the collections field to
   each Record in the BTE pipeline.

   Important note: an entry with key "collections" should be in the
   output map that is used by the DSpaceOutputGenerator.