hepdatautils: incremental harvest support

 * Adds parameter --starting-date.
 * Stores the last harvest and reuses it as starting point if
   no starting date is provided.

Co-authored-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>