drivers/mks_g_series.py many updates: Python 3 only, NAK support, com…

[Ejler]

Updates to mks_g_series.py driver

• Clean up comm() and checksum(). Now supports Python 3 only (the try: except UnicodeDecodeError to only be a problem in Python 2)
• The previous commit only works if zeros (noise) are added before/after the MFC response (as was the case on the microreactor setup). A few checks now take care of noise in the comms without hardcoding too much
• Added NAK support with error message table
• Added function: read_internal_temperature
  The driver has been tested on the original sniffer setup (Python 3.9.2) and the original microreactor setup (Python 3.5.3) and works consistently on both of them (whereas before it didn't).

[Ejler]

Will close PR #19

[KennethNielsen]

I found no problems in this, but I also do not have the manual next to me, so consider this just a comment.