Add testing framework for Modbus RTU

[brainelectronics]

Added

• Fake machine module with UART and Pin class to be used on Unix MicroPython container for RTU tests and examples, see #47
• RTU host example script
• RTU docker compose file and RTU docker compose file test based in MicroPython 1.18 image
• RTU client Dockerfile and RTU host Dockerfile based on MicroPython 1.18 image
• Initial RTU examples unittest
• RTU example section for Client and Host in USAGE

Changed

• Outsourced the following common functions of serial.py and tcp.py into CommonModbusFunctions of common.py:

  • read_coils
  • read_discrete_inputs
  • read_holding_registers
  • read_input_registers
  • write_single_coil
  • write_single_register
  • write_multiple_coils
  • write_multiple_registers

• Inherit from CommonModbusFunctions in Serial of serial.py and in TCP of of tcp.py

• Extended RTU client example for Docker usage to load all registers from example JSON file

• Update internal functions parameter name from slave_id to slave_addr of TCP's _create_mbap_hdr and _validate_resp_hdr function to be the same as in Serial

• Update Modbus function documentation from TCP specific to common module in USAGE file

• Renamed docker files:

  • Dockerfile.client -> Dockerfile.client_tcp
  • Dockerfile.host -> Dockerfile.host_tcp
  • Dockerfile.test_tcp_example -> Dockerfile.test_examples