Change endpoint to not include version

[dblock]

As is, the README for stations API will not work because you need to use the api.openweathermap.org/data/3.0 endpoint.

I propose to add api_root (e.g. https://api.openweathermap.org), api_path, e.g. data, and api_version, then combine these into endpoint.

Then document that this now supports 2.5 and 3.0 endpoints.

[dblock]

I did something simpler in #23, so you don't need to specify endpoint to use the stations and measurements APIs.

[wasabigeek]

🙏