Allow grid meter to get inverted

[goebelmeier]

My grid meter is currently providing positive values as long as power gets delivered to the grid and negative values if power is getting purchased from the grid.

Please implement a config parameter to invert the reading. Currently i'm just dry-running evcc just with my meters. Charger and vehicle is configured as dummy script.

[andig]

Would you expect this on the plugin or on the meter level? I see the same issue with the total energy- kWh vs Wh.

Note: the MQTT provider already supports this.

[goebelmeier]

From my point of view, it should be safe on the meter level

[andig]

@goebelmeier as per #90 the scale parameter is supported for HTTP and MQTT devices. Where else would we need this? Modbus? Websocket?