[wcurrangroome]

Closes #22. Stacked on #23 (household weights); review that one first.

What this changes

• Same-year crosswalks for years 2000–2009 now come from Geocorr 2000, which uses 2000 Census geography. Example: 2000 tracts to 2000 PUMAs, which NHGIS does not publish. Years before 2000 are still an error.
• The package queries Geocorr 2000 directly. It uses the same address and form fields as Geocorr 2018, plus one empty uaucs field that Geocorr 2000 requires.
• Supported geographies: block, block group, tract, county, place, ZCTA, puma (meaning puma5, the 2000 5% sample PUMA that 2005–2011 ACS microdata also use) and msacmsa (1999 MSAs and CMSAs). Asking for puma1 stops with a message pointing to puma5, because Geocorr 2000 returns no rows for it.
• Geocorr 2000 leaves the MSA/CMSA code blank for New England, where 1999 metro areas were built from towns. Those 3,204 rows are dropped with a warning.
• Changes to the shared Geocorr download code:
  • it finds the CSV link in either the page text or the page's links;
  • it stops with a clear message when Geocorr returns zero rows;
  • it uses each version's limit on states per block request (10 for 2000, 13 for later versions).
• 11-character tract GEOIDs are built from the county code and Geocorr 2000's tract codes, which are written like 1234.56.
• get_available_crosswalks() lists the 56 new combinations.

Decisions

• Keep "msacmsa" as the name. 1999 PMSAs are not added.
• For 2000, "puma" means puma5.
• New England rows with no MSA/CMSA code are dropped with a warning.
• No reader for hand-downloaded Geocorr 2000 CSV files, because automated queries work.

Testing

• A live national request for tract → PUMA (2000) returned:
  • 2,071 PUMAs, the published count;
  • 50 states plus DC;
  • 11-character tract GEOIDs and 7-character PUMA GEOIDs;
  • a total population of 281,421,916;
  • allocation factors that sum to 1 for each tract.
• test-get_geocorr_2000_crosswalk.R: all tests pass, including the 4 live ones. Run them with NOT_CRAN=true CROSSWALK_RUN_SLOW_TESTS=true.
• test-get_geocorr_2018_crosswalk.R: 0 failed. One test that expected year 2000 to fail now uses 1990.
• Live checks that the Geocorr 2022 and 2018 requests still return correct GEOID lengths after the shared download code changed.
• devtools::check() without tests and vignettes: 0 errors, 0 warnings.

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