Release 0.6: faster connecting to new device and support for custom hardware and commands

* added product_id property to help identify hardware variants
* I2C: fix communication when using smbus2 library
* doc: overview of hardware variants
* new hardware variants: ECU-PCON-mp6single, ECU-PCON-mp6quad, ECU-PCON-ABP2LAN and ECU-PCON-SLF3
* improved connecting to new device, this speeds up the process significantly
* added `send_raw_command` to allow sending custom commands and parsing the response
* `channel_info`: it now also return high and low side output voltages
* serial: clear buffers before sending command
* refactored parsing response data
* refactored definition of hardware variants