Add support for onchip temperature

[tuupola]

Should look something like:

from machine import I2C, Pin
from mpu9250 import MPU9250

i2c = I2C(scl=Pin(22), sda=Pin(21))
sensor = MPU9250(i2c)

print(sensor.temperature)

[vvkuryshev]

Hello, Mika!

Great project. It works on esp8266 (I use acceleration and rotation measurement). But in fact temperature is not added to mpu9250 module.

[tuupola]

Yeah temperature is still on TODO list. I should pay attention to this library again in the near future.