Add multi-station weather querying

[jdemaeyer]

If one of the meteorological elements is not available from our preferred station, get it from the next one.

I.e. instead of

SELECT DISTINCT ON (timestamp)
    temperature,
    ...

do

SELECT
    timestamp,
    FIRST(temperature),
    ...
GROUP BY timestamp

This implies that we cannot return source information (station_id, location, etc.), since a single timestamp could be comprised of elements from multiple sources.

[jdemaeyer]

Benchmarks:

By lat/lon:
  50 one-day queries,  sequential:   2.76 s
  50 one-day queries,  parallel:     1.53 s
  50 one-week queries, sequential:   7.45 s
  50 one-week queries, parallel:     4.95 s

By lat/lon, no fallback:
  50 one-day queries,  sequential:   2.12 s
  50 one-day queries,  parallel:     1.16 s
  50 one-week queries, sequential:   5.70 s
  50 one-week queries, parallel:     3.77 s

By station:
  50 one-day queries,  sequential:   1.30 s
  50 one-day queries,  parallel:     0.59 s
  50 one-week queries, sequential:   3.42 s
  50 one-week queries, parallel:     1.64 s

By source:
  50 one-day queries,  sequential:   0.53 s
  50 one-day queries,  parallel:     0.25 s
  50 one-week queries, sequential:   2.11 s
  50 one-week queries, parallel:     1.00 s