New code for fetching source datasets

[Nick Barnes]

In updating ccc-gistemp to fetch GHCN 3.1.0 (and to stop fetching USHCN), I inadvertently made a clean spot in fetch.py, so then I had to throw it all away and start again.  I hate it when that happens.  Please say farewell to tool/preflight.py and welcome the all-new tool/fetch.py and the configuration file config/sources
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Nick Barnes, Climate Code Foundation, http://climatecode.org/