Add an option "ntuple_file" like "histo_file" to enable NtupleSvc?

The following discussion from !347 (merged) should be addressed:

• @sstahl started a discussion: (+1 comment)

  @rmatev , @apearce. Should we add an option "ntuple_file" like "histo_file" which enables the service? Mainly making a comment to create an issue.