Modbus RTU: support single queries to devices

Currently Modbus can be configured by an address and datatype and scan interval. This causes probes for each address. It is also possible to query a range of addresses, this would mean that for a single slave a definition of its addressspace would be enough to retrieve everything under it and therefore reduces the line consumption. Sure I could write my own Modbus RTU implementation and put it on MQTT, but I think this may be something that would benefit users generically.