Add inputs for tax variables about policyengine-us HOT 5 CLOSED

1. create_units() is processing a given data, but the example class created in survey_input_variables.py are only defining some attributes. Where should the processing logic go?
2. where to look up the meaning of those keys in create_units() data, such as a_spise, s_flag, etc?
3. where to look up definitions in raw data cps.csv? Most variables there are named with numbers, do I keep that convention or rename them to something meaningful?
4. There is cps.csv data and the spm uk data. for now, I will only need the first one? So is cps.csv data?
5. Relationship between all ongoing projects and involved repos

(This is just for my own notes, still reading doc/about_cps_data.md)

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Relevant: FPUS notebook

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1. Didn't see P_INT_VAL in raw_cps_2020.h5?
2. BaseCPS syntax

Notes:
command to download the raw_cps_2020.h5:
openfisca-data raw_cps generate 2020
The h5 file should be located at openfisca_data/microdata/external

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@moon-home the P_ prefix is added to indicate the variable is at the person level (the raw dataset is a relational database with three tables: person, family, household). See this part of dataset.py for the code adding it.

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All now added

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