Refactor wisqars loading function

[benhammondmusic]

Description and Motivation

• closes Should we abstract def load_wisqars_df_from_data_dir #3317
• creates new util function to handle the actual loading of the csv
• renames per data source processing functions; I found it confusing that each source had it's own version of load_ function but they each did their own things
• adds types to refactored functions
• fixed issue with data file naming once it was caught by the new function usage
• adds script to run mypy locally with the same settings that will run on CICD. just run ./scripts/run_mpyp.sh from the terminal and it'll type checked any changed python files on your local branch

Has this been tested? How?

• tests still pass

Types of changes

(leave all that apply)

• Refactor / chore

New frontend preview link is below in the Netlify comment 😎