Added sensor prefix option and added prefix to sensor names published to home assistant.

Added option for sensor prefix

Forgot the actual function.

Confused files. No correct.

Changed if statement to use proper test syntax ("$STRING" = "STRING" not $STRING == "STRING").

Home Assistant warning about not allowed characters in discovery topic resulting in characters other than letters or _ in the SENSOR_PREFIX. Added sed to replace any character other than letter or _ with _ and remove double occurrences.

Letter, numbers and _ are kept of cause. Added conversion from upper to lower case in SENSOR_PREFIX function.