De-glitch temperature/other readings.

The temperature (and doubtless other) readings show occasional insane glitches. Once we get enough data in the can (a few weeks with a few boards), either fix the source problem or add deglitching, or both.

The worry is that the non-RT nature of python-on-pi might trigger worse problems (setting things over the SPI bus). If it is just occasional bad readings we are OK.