Raspberry pi3B Python i2c pcf8574 keypad16
Original code from http://www.mikronauts.com/raspberry-pi/raspberry-pi-4x4-keypad-i2c-MCP23017-howto/
picture1 from http://qqtrading.com.my/keypad-button-4x4-numeric-matrix-array
picture2 from https://nettigo.eu/products/pcf8574-module-8-bit-i-o-expander-for-i2c-bus
picture3 from https://hexdocs.pm/nerves_system_rpi3/1.0.0-rc.0/readme.html