Add next_slot_available convenience method

Author: dericke

overpass/api.py

@@ -4,6 +4,7 @@
 # See LICENSE.txt for the full license text.
 
 import csv
+from typing import Union
 import json
 import logging
 import re
@@ -200,6 +201,15 @@ def slots_running(self) -> tuple:
         """
         return self._api_status()["running_slots"]
 
+    @property
+    def next_slot_available(self) -> Union(None, datetime):
+        """
+        :returns: None if a slot is available now, or a datetime for when the next slot is free
+        """
+        if self.slots_available:
+            return None
+        return min(self.slots_running + self.slots_waiting)
+
     def search(self, feature_type, regex=False):
         """Search for something."""
         raise NotImplementedError()