Add access to control heater_level

[int3ll3ct]

Add the ability to control heater_level with the thermostat feature turned off.

The heater_level concept in freshroastsr700 is powerful in that it allows finer control over heat application than the hardware implementation - namely, by default, heater_level can be varied from 0 to 8 for heat application on a linear scale, whereas the heat settings of Lo Med Hi in the SR700 are tuned to reach specific chamber temperature targets. In some scenarios - specifically, when attempting to control the SR700 using an external software PID (say, with Artisan...) - it would be useful to be able to drive the heater_level directly from higher-level software.

[int3ll3ct]

Implemented in v0.2.4