feat: add save many to filedb

Author: Lauric Desauw

powerapi/database/file_db.py

@@ -144,4 +144,5 @@ def save_many(self, reports: List[Report]):
 
         :param reports: Batch of data.
         """
-        raise DBError("FileDB do not support save_many method")
+        for report in reports:
+            self.save(report)