Backend: adds state maternal counts, ALL per 100k

[benhammondmusic]

Description and Motivation

closes #3535

Data Source

• clarify constant strings from sources as JAMA_ or CDC_
• clarify merge_state_counts vs merge_national_counts functions which load from CDC or JAMA respectively

CDC Natality

• ingests live births from CDC, same data used by study to calc per 100k
• adds doc string with how-to for replicating the download process

Linting

• handle some linting issues with "sum" agg() function

Data Loading

• adds load_tsv function

Testing

• updates tests to read actual data/ rather than fake test input, via mocked redicrect to data/
• deletes test input data files
• updates util fn to only use whichever kwargs are supplied

Has this been tested? How?

• golden data updated to include ALL state level
• tests passing

Types of changes

(leave all that apply)

• New content or feature

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