Initial stab at harvester for sensor.community.

This uses a single daily request to the static data API to derive a dataset for each sensor which produced data in the last 24 hours.

There are quite a few rough edges:

• We do not have a per-sensor landing page but point every dataset at sensor.community's homepage.
• We do not know the time range for which data is available via the sensor.community's archive for any given sensor ID. Figuring this out via downloads might place undue load on their servers.
• Our decoder ring for the measured values is still incomplete. Specifically, the following ones still lack a translation:
  • counts
  • counts_per_minute
  • durP1
  • durP2
  • height
  • hv_pulses
  • lat
  • lon
  • max_micro
  • min_micro
  • N05
  • N10
  • N1
  • N25
  • N4
  • P0
  • P4
  • pressure_sealevel
  • ratioP1
  • ratioP2
  • samples
  • sample_time_ms
  • timestamp
  • TS