[wcurrangroome]

Closes #20.

What this changes

• get_crosswalk() and crosswalk_data() accept weight = "household", "housing_owned" and "housing_rented". For NHGIS block group and tract crosswalks, these select the NHGIS wt_hh, wt_ownhu and wt_renthu weights.
• Every step of a crosswalk plan is checked before anything is downloaded. Asking a GeoCorr step for one of the new weights raises an error that names the step (for example, "step 2 of 2") and lists the weights GeoCorr offers: population, housing and land.
• An NHGIS file that lacks a requested household or tenure weight raises an error that lists the weights the file has. Another weight is never substituted. Block-based files are expected to fall in this group.
• CT Data (2020↔2022) and county-events steps have no choice of weight. They accept any valid weight and print a message when they use population-based factors instead.
• crosswalk_metadata$weighting_variable records the weight actually used, using NHGIS's own label. For example, weight = "housing" on a tract file now records "housing_all"; the value the user passed is still in call_parameters$weight.
• The help pages and the interpolation vignette list the weights each source offers.
• The six tests on the CT Data 2020→2022 path now skip when IPUMS_API_KEY is not set, because that path downloads an NHGIS file.

Decisions

• No "adult" or "family" weights for now.
• get_available_crosswalks() gets no new column; the available weights are only documented.

Testing

• test-crosswalk-weights.R: 16 tests, 0 failed, 4 skipped. The skipped ones need IPUMS_API_KEY or slow tests turned on.
• test-get_crosswalk.R: 0 failed. test-crosswalk_data.R: 0 failed.
• Not yet confirmed against a real NHGIS file: that the tract and block group files carry wt_hh, wt_ownhu and wt_renthu. No API key was available. Those tests run when IPUMS_API_KEY is set.

Stacked pull requests: #22's PR is based on this branch, and #21's PR is based on #22's.

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