Improve OGE validation compared to eGRID

[grgmiller]

This PR improves and automates the validation of OGE output data compared to eGRID data (fixes CAR-1893, CAR-1894, CAR-1892, CAR-1887).

Updates:

• update download_data.download_egrid_file() to only download the single eGRID file for the year for which the pipeline is being run. Previously, we had downloaded all eGRID files 2018-2021. Note: until we start expanding historical coverage, I am not planning to add urls for eGRID versions prior to 2018.
• Moves all of the egrid validation functions from src.validation.py to a new file src.validate_egrid.py - we might want to name this something different?
• Creates a new constants.py file to hold constants referenced across the repo (prevents circular imports)
• Started work on a new automated logic to identify why discrepancies exist between eGRID data and OGE data

To do:

• check that all eGRID column mapping / loading is correct
• Calculate and/or load _for_electricity and _for_electricity_adjusted values
• Make dataframe filters more legible when filtering plant data errors
• When there is a mismatched set of plants, examine how much of difference results from mismatch, then if there is remaining difference for matched set of plants
• When flagging issues, tag how many plants have that issue so that we make sure we are capturing all issues and there are no problem plants that are uncategorized.
• Consider changing plant comparison status to a numeric percent instead of text category (or have both).

Where to pick up work next time (3/11/23): started transferring code to a fresh notebook to put it all in order. Need to re-build the plant comparison df