Add "allow_position_stacking" value to config, which allows rebuys of a pair

Add function unlock_reason(str: pair) which removes all PairLocks with reason
Provide demo strategy that allows buying the same pair multiple times

freqtrade/configuration/configuration.py

@@ -137,6 +137,12 @@ class Configuration:
         setup_logging(config)
 
     def _process_trading_options(self, config: Dict[str, Any]) -> None:
+
+        # Allow_position_stacking defaults to False
+        if not config.get('allow_position_stacking'):
+            config['allow_position_stacking'] = False
+        logger.info('Allow_position_stacking is set to ' + str(config['allow_position_stacking']))
+
         if config['runmode'] not in TRADING_MODES:
             return