Changes the default behavior of creating new child descriptions for descriptions
that already have a digital object

When importing digital objects via the command line, the default behavior is to create
new child descriptions when multiple lines point to the same description. However,
there may be cases where the user has mistakenly duplicated identifiers and new descriptions
start to pollute the database. Therefore, not adding the --attach-only option has the
same effect as adding it.

Changing this, the default behavior is now to inform the user about duplicate lines
that point to a description that already has a digital object, indicating that a
digital object already exists for that identifier. If the user wants, he can use
the --attach-only option to change the behavior of the task that will add child
descriptions to attach the digital object.