Modbus: Entity-based default functions

[egekorkan]

Currently, the default modbus functions are defined for WoT operations (op keyword). However, in practice, they are more defined based on the entity and the op keyword. So if there is readproperty and Coil, then the function is readCoil. I think this should be in the Modbus binding as the way of defaulting. There is one odd case that a Coil can be writeMultipleCoils or writeSingleCoil. Maybe in that case, the default function should be derived from the quantity in the URL.