Add read/write capabilities to modbusclient

I updated the "smbclient" module to allow reading and writing data to PLCs (Programmable Logic Controllers) using the ModbusTCP protocol.
Previously it was possible to write data to a register.
Now it is possible to read and write data from both coils and registers.

The new code is not based on the previous module but rather on "modicon_stux_transfer" from Digital Bond.

I tested it against the "modbuspal" simulator and a real Schneider m340 PLC.