Support string IDs for participants and statements

[patcon]

Reticketed from Polis User Group discord message by @nicobao: https://discord.com/channels/815450304421691412/1355624576159318199/1394451430269779979

Currently, some places in the codebase assume IDs are numeric like they are for upstream polis. It shouldn't be too hard, but string IDs should be accepted into run_pipeline()

Todos

• improve this ticket to be "good first issue"
• add clearer todos

[patcon]

I'd say step one would be to write a simple test that sends a variant of an existing fixture into run_pipeline(), but the variant should be preprocessed to have strings as participant IDs (can probably just convert each int into a string, and append the letter "p" so it can't be recast to an int without error).

The test should just allow running without errors, and maybe a sanity check of 1 or 2 calculated values coming out, to make sure they match what's expected from math_data. You can copy these sanity checks from some other test. Look for an existing test exercising as much of run_pipeline as you can find, and then preprocess its loader.math_data. Once it's failing, you can fix the failures that should crop up. After they're fixed, you can just copy the minimal version of that original test into it's own test :)

Thanks again @pivilartisant!