Allow site-wide configuration via environment variables for data directory containing standard data.

[aerorahul]

Rationale

Allow easier site-wide use of standard data (such as that from NaturalEarth).
Addresses Issue #1281

Cartopy allows for site-wide customizations via cartopy.config and siteconfig.py, but that requires system admins to place this file into the installation directory. Cartopy also allows for users to configuration via cartopy_userconfig, but most users are not savvy about such things.

Implications

This will allow site-wide use of system installed/provided standard data as well as users to provide their own path to this data via an environment variable CARTOPY_DATA_DIR.

Checklist

• CLA -- Signed

• Example:

export CARTOPY_DATA_DIR="absolute path to a directory where standard data can be found"

• Ensure there is a suitable item in the cartopy test suite for the change you are proposing -- N/A

[greglucas]

Seems reasonable to me and the env variable seems like a sensible name.

[greglucas]

Thanks, @aerorahul!