Fix OpenAQ reader

[zmoon]

• local time calc

  • don't quite understand why it wasn't working properly (giving only a few unique UTC offsets), but looks more reasonable now (several unique UTC offsets and correct e.g. most common is +8 UTC which mostly corresponds to sites in China)

• convert to ppmv

  • there was code to do so but it wasn't being used
  • updated and documented the conversion factors and added factors for CH4 and NO which can be in OpenAQ data

• [x] at least one test (maybe find a way to set things to load just one file for testing)

[zmoon]

Some initial timings suggest we can gain speed by using s3 URL scheme instead of current HTTPS:

In [8]: %timeit -n 10 pd.read_json("https://openaq-fetches.s3.amazonaws.com/realtime/2013-11-26/2013-11-26.ndjson",
   ...:  lines=True)
311 ms ± 37.1 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [9]: %timeit -n 10 pd.read_json("s3://openaq-fetches/realtime/2013-11-26/2013-11-26.ndjson", lines=True)
159 ms ± 13.5 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

similar to #113