Add low pass filter to barometer temperature data

[KoffeinFlummi]

We had some issues with spikes in the barometer readings recently. Those were mostly caused by spikes in the temperature readings from the barometer (either single values or small series of values), which then cause large changes in the pressure output due to the temperature compensation.

It should be possible to low-pass filter the temperature readings, either with a simple IIR filter or something more involved.