OpenAQ v1 fixes + initial OpenAQ v2 reader

[zmoon]

Closes #130 #122

[bbakernoaa]

I would just make sure we document where to receive the API key

[zmoon]

Thanks @bbakernoaa . I have this automatic warning set up:

monetio/monetio/obs/openaq_v2.py

Lines 60 to 67 in 4490a50

	if API_KEY is None:
	warnings.warn(
	"Non-cached requests to the OpenAQ v2 web API will be slow without an API key, "
	"or might fail (HTTP error 401). "
	"Obtain one (https://docs.openaq.org/docs/getting-started#api-key) "
	"and set your OPENAQ_API_KEY environment variable.",
	stacklevel=2,
	)

but I will also add the relevant info to the module docstring.