create global list of scraper IDs to be used in compare.py, streamlit_app.py, and runall.py

[lorae]

Currently, there are three lists of scraper IDs, each contained in compare.py, streamlit_app.py, or runall.py, which look like the following:

source_order = ['NBER', 'FED-BOARD', 'FED-BOARD-NOTES', 'FED-ATLANTA', 'FED-BOSTON', 'FED-CHICAGO', 'FED-CLEVELAND', 'FED-DALLAS', 'FED-KANSASCITY', 'FED-NEWYORK', 'FED-PHILADELPHIA', 'FED-RICHMOND', 'FED-SANFRANCISCO', 'FED-STLOUIS', 'BEA', 'BFI', 'BIS', 'BOE', 'ECB', 'IMF']

Integrating a new scraper module in the project involves updating all three lists, which is unintuitive and prone to producing bugs.

Resolving this issue would involve creating a global order of sources that can be called by all 3 scripts.