Implement offsets for YieldTotal (#549)

This allows to enter a offset in kWh in the inverter properties which will be applied to the read Yield Total value of the inverter. Using this can set your total production to zero if you e.g. are using a used device.

src/Configuration.cpp

@@ -100,6 +100,7 @@ bool ConfigurationClass::write()
             JsonObject chanData = channel.createNestedObject();
             chanData["name"] = config.Inverter[i].channel[c].Name;
             chanData["max_power"] = config.Inverter[i].channel[c].MaxChannelPower;
+            chanData["yield_total_offset"] = config.Inverter[i].channel[c].YieldTotalOffset;
         }
     }
 
@@ -228,6 +229,7 @@ bool ConfigurationClass::read()
         JsonArray channel = inv["channel"];
         for (uint8_t c = 0; c < INV_MAX_CHAN_COUNT; c++) {
             config.Inverter[i].channel[c].MaxChannelPower = channel[c]["max_power"] | 0;
+            config.Inverter[i].channel[c].YieldTotalOffset = channel[c]["yield_total_offset"] | 0.0f;
             strlcpy(config.Inverter[i].channel[c].Name, channel[c]["name"] | "", sizeof(config.Inverter[i].channel[c].Name));
         }
     }