Fetch SSPUpdate data for v3.2 release

[khaeru]

@lukeggert helped me notice that there is finally a version of the SSP 'driver' data (including population and GDP) publicly released, and not gated behind a registration link: https://ssp.apps.ece.iiasa.ac.at/documentation/basic-drivers

To take advantage of this, the SSPUpdate class can be improved to:

• Read and process this version, by default.
• Fetch and cache the version from the above URL, or from Zenodo.
• Only optionally require the message-static-data files in the local data directory.

The 'fuzzed' version(s) of the data in the test suite can then probably be discarded, as this was only intended to be a public substitute for the embargoed pre-release versions of SSP 3.x.