temperature readout

[scboulogne]

I have added and tested the temperature readout on this sensor.

This required changes in Adafruit_LIS2MDL.h and .cpp for the new public method
float Adafruit_LIS2MDL::readTemperature(void);
see diffs for my version of these files below (sorry, as .txt because for all my attempts, they were refused to get attached with extension .patch as unrecognized file types ...).

NB: this is similar to float Adafruit_MMC5603::readTemperature(void); but much simpler because the temperature registers on LIS2MDL are refreshed automatically

NB: it is possible (see comments in the .cpp) to store the result in the fourth (unused) element of event.data but it is unclear to me whether that may interfere with smth else.

Adafruit_LIS2MDL-h.txt
Adafruit_LIS2MDL-cpp.txt