Implement fixes of PR #95

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[gijsio]

Adding to the description in #95, the branch includes 2 new API calls to enable / disable the internal heating element of the temperature sensor (SI7006A20). This can be used to temporarily heat up the sensor in order to remove any condensation and can be used as follows:

# py = Pycoproc
si = SI7006A20(py)
si.heater_control(False) # disable heater
time.sleep(10)
print(si.temperature()) # returns ~20*C
si.write_heater_reg(15) # set maximum heater current
si.heater_control(True) # enable heater
time.sleep(10)
print(si.temperature()) # returns ~45*C
si.write_heater_reg(1) # set low heater current
time.sleep(10)
print(si.temperature()) # returns ~30*C

Another API call for the light sensor (LTR329ALS01) is the calculation of illuminance in lux, implemented according to Appendix A of the datasheet (not publically available)

# py = Pycoproc
lt = LTR329ALS01(py)
print("Light (channel Blue lux, channel Red lux): " + str(lt.light()), str(lt.lux()))