When running the `examples/read-all.py` script I was seeing a crash because
the value passed to `write_i2c_block_data` wasn't being converted to an int:
```
File "/home/pi/tmp/bme680/library/bme680/__init__.py", line 281, in _set_regs
self._i2c.write_i2c_block_data(self.i2c_addr, register, value)
TypeError: Third argument must be a list of at least one, but not more than 32 integers
```
This commit ensures `set_gas_heater_temperature` converts the temperature to an int
before it's passed to `self._set_regs`.
