Support Modbus sensors

[dc42]

We should support temperature, humidity etc. sensors that communicate via Modbus. Examples:

• https://www.amazon.co.uk/Temperature-Humidity-Precision-Transmitter-Compatible/dp/B0836J2J2C. I have one of these. It supports Modbus function 4 to read register 1, register 2 or registers 1 and 2. Temperature is returned in register 1, humidity in register 2.
• https://uk.robotshop.com/products/industrial-grade-modbus-rs485-temperature-humidity-sensor
• https://www.calex.co.uk/product/temperature-measurement/infrared-temperature-sensors/pyrominibus/

We will need to specify at least the following:

• Which serial channel to use for Modbus comms (typically the P parameter)
• The slave address of the sensor (can't use A parameter, that's already used for the sensor name)
• Which function code to use (normally 3 or 4)
• Which register to read and how many registers to read. Can we assume it's always 1 register per output?
• Which register provides the required value - maybe the first register for the primary sensor, then any additional registers read are for additional outputs?
• What scaling factor and offset to use (need to provide these for each output, if supporting multiple outputs)
• The minimum interval required between successive reads (some sensors need this, so we should support multiple outputs)

[dc42]

I already added the framework for supporting virtual ports in RRF (similar to the existing code in Duet3Expanson) so we can add virtual Modbus ports in future.