Handle low temperature

[albireox]

From Luzius:

A series of testing the positioners at negative temperatures showed some issues. In order to guarantee the correct operation of the positioners
in a very cold environment we propose two measures:

1. limit the maximum motor speed to 3000 rpm at negative temperatures (current requirement: 5000 rpm)
2. And in addition, heat the motors with an additional power of 0.7 watts (total idle power per heated positioner: 1.2 watts) at temperatures below -10 deg C.

More details are on the wiki:
https://wiki.sdss.org/display/FPS/Negative+Temperature+Test+Results

[albireox]

Depends on #109 because we need a callback on one of the sensor temperatures (probably RTD2) to trigger the low temperature mode.