Clarify HERE's stddev calculation

[Nate-Wessel]

What this pull request accomplishes:

• clarifies some ambiguity in the stddev field for HERE data

Issue(s) this solves:

• It was ambiguous whether this was the sample or population metric because there are 0's, where there should be NULLs.

What, in particular, needs to reviewed:

• facts

What needs to be done by a sysadmin after this PR is merged

NA

[Nate-Wessel]

As proof:

SELECT
    link_dir,
    tx,
    stddev,
    stddev_samp(obs.speed),
    stddev_pop(obs.speed)
FROM here.ta_path, 
LATERAL (SELECT unnest(ARRAY[min_spd,max_spd]) AS speed) AS obs
WHERE sample_size = 2
GROUP BY
    link_dir,
    tx,
    stddev
LIMIT 100