Add method to disable gas heater

[graeme-winter]

Setting either of temperature or heat time to zero will disable

Fixes adafruit/Adafruit_CircuitPython_BME680#63

[tekktrik]

Is this ready for review?

[graeme-winter]

I'll welcome some review comments, not had a chance to polish though

[caternuson]

I'd suggest using None to disable instead of 0. That'd be similar to what is done in the Motor library for disabling servo outputs: https://github.com/adafruit/Adafruit_CircuitPython_Motor/pull/27