Add Friedland datasets to cl.load_sample()

[genedan]

Closes #561. The Friedland datasets have been sitting around in the utils/data folder for quite some time, but haven't been loadable by cl.load_sample(). This PR enables that, updates the documentation, and also adds a unit test to make sure all of the supported datasets can load.

I've also added cumulative claim columns to friedland_us_industry_auto_case and friedland_xyz_case . These datasets previously had mixed cumulative/incremental columns which would be incompatible with the Triangle class. This changes those samples to be just cumulative. Therefore, load_sample() won't be 1-to-1 with the text, but the old columns can still be derived and still remain in the original CSVs.

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Note

Medium Risk
Medium risk because it expands load_sample() dataset-specific parsing rules and changes/removes several sample CSVs, which could break downstream users relying on prior sample names/column schemas.

Overview
Adds support for loading the previously bundled Friedland sample datasets via cl.load_sample() by introducing dataset-specific column/origin/development mappings (including semiannual friedland_auto_freq_sev handling) and improving the invalid-key error message.

Updates sample CSVs to align with Triangle expectations by adding cumulative Paid Claims columns to friedland_us_industry_auto_case and friedland_xyz_case, adjusting friedland_auto_freq_sev half-year labels, and removing three older friedland_uspp_*increasing* CSV variants.

Extends test coverage with a new unit test that iterates over all files in utils/data and asserts each can be loaded, and updates the sample-data documentation table to list the Friedland datasets and their attributes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.25%. Comparing base (a020797) to head (2932f75).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
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+ Coverage   85.13%   85.25%   +0.11%     
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  Files          85       85              
  Lines        4911     4950      +39     
  Branches      630      645      +15     
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+ Hits         4181     4220      +39     
  Misses        521      521              
  Partials      209      209              

Flag	Coverage Δ
unittests	85.25% <100.00%> (+0.11%)	⬆️

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[henrydingliu]

do we need to update manifest.in as well? i'm still not able to load_sample friedland locally