Feed positions analyser

A brief summary of changes that would be good to have for the feed positions analyser:

Grafana:

- [ ] the singlestats should show only the last 24 hours, but seem to vary depending on the time range selected.
- [ ] it would be nice to have a good/bad stat that summarise whether the data is good or not.

Analyzer:

- [x] at the moment the good/bad thresholds are actually hardcoded in the analyzer. This is a policy decision that's probably better at a higher layer. We should expose relevant statistics into prometheus and let those decisions be made higher up. The key question is what are the statistics to export, possibly `num_feeds_above_threshold{threshold="1.0",freq_id="14"}`, i.e. we pick a few (maybe ~10) distance thresholds and just return the number above each.

Analysis:

There's a few questions we need to address from the analysis:

- [ ] when is a feed good/bad?
- [ ] when is the instrument good/bad? What statistic do we test, and what threshold should we use?
- [ ] look back at historical data to see if/when/how this analyser is catching periods of bad data? There are certain failures it should be immune from (i.e. calibration/flagging), but it should catching rain event jumps, decorrelated cylinders, massive RFI issues etc.