added setHeaterLevel function

[skymaker-c2is]

I had this modification lying around on my disk for a while. I'm using it since ~10 months now and it's working. I needed to set the heater level of the Si7021, since the default setting isn't suitable for "baking" the sensor free after a long duration of high humidity. As I'm not a professional software developer, please be kind and have mercy if something's wrong - also, this is my first pull request. ;)