Migrate options() to environment variables as default

[Jens von Bergmann]

Right now we set the API key and cache path via options. It might be cleaner to set these as environment variables by default instead. Nothing would change in terms of user experience, but we would change the defaults in the function calls and also change the docs, but fall back to options in case environment variables aren't set. And also change the docs and vignette accordingly.

[Jens von Bergmann]

Addressed in https://github.com/mountainMath/cancensus/tree/intersect_geometry2

[Jens von Bergmann]

Done and merged into main.