OpenAQ fixes

[zmoon]

Aside from the plan to increase the speed (#122), while working on an MM CLI for @jordanschnell I found some things that should be fixed in the current OpenAQ reader that uses openaq-fetches.

• returned df currently may have some times outside of the requested dates (i.e. greater than the ub (in OpenAQ v1 fixes + initial OpenAQ v2 reader #141)
• returned df may have non-unique site IDs at given time.
  Seems like there are two things going on here:
  • lat/lon that are only slightly different, the precision of the lat/lon in the current site IDs doesn't capture. Maybe create new site IDs, e.g. country code + hash of full-precision lat/lon (in OpenAQ v1 fixes + initial OpenAQ v2 reader #141)
  • lat/lon are the same but different measurements in different rows (e.g. 2 rows). Should combine these into one row where possible.
• typo in BC's new variable name with units (fixed in 01e14d3)
• add some tests which load a reduced set of files (in OpenAQ v1 fixes + initial OpenAQ v2 reader #141)