Clean stats before start computing new ones

Signed-off-by: Marco Marinello <mmarinello@sezf.it>

stats/management/commands/update_stats.py

@@ -12,6 +12,7 @@ class Command(BaseCommand):
     help = _('Updates the daily used computers statistics for the chosen day, aggregated by schools.')
 
     def handle(self, *args, **options):
+        stmodels.ComponentsStat.objects.all().delete()
         how_many_schools = 0
         how_many_components = 0
         how_many_global = 0