gpsd extension fails occasionally #581
Description
The gpsd extension fails occasionally, leaving gaps in the record/plots.
Debugging indicates this is because the grep sub shell command sometimes fails and returns a back trace to snmp, instead of the anticipated JSON. Specifically, the grep for altitude sometimes returns a line with "health" in it instead of "alt", and this results in an error when trying to extract the altitude value.
Fix: 1) add grep option -w to search on word boundaries. 2) send python part of pipe stderr to /dev/null
PR to follow.