Strip leading zeros from CEMS emission_unit_id_epa

[grgmiller]

Purpose

We had noticed that in 2023, the PJM power sector results file had a missing fuel category. I traced this to plant 50852, and it turns out the cause of the issue was that the fuel type backfill using the PSDC was failing because we were not stripping leading zeros from the unit ID when loading the table, so the merge was not working.

After digging into this more, I realized that pudl does not automatically strip leading zeros from emissions_unit_id_epa (see catalyst-cooperative/pudl#3992). This PR adds functionality to always strip these leading zeros whenever loading CEMS data

This PR also updates the sandbox notebook to use 2023 as the year, and also adds some starter functions for loading intermediate output data for exploration since these frequently need to be loaded.

Testing

Ran pipeline for 2023

Where to look

• It's helpful to clarify where your new code lives if you moved files around or there could be confusion/

• What files are most important?

Usage Example/Visuals

How the code can be used and/or images of any graphs, tables or other visuals (not always applicable).

Review estimate

How long will it take for reviewers and observers to understand this code change?

Future work

What issues were identified that are not being addressed in this PR but should be addressed in future work?

Checklist

• Update the documentation to reflect changes made in this PR
• Format all updated python files using black
• Clear outputs from all notebooks modified
• Add docstrings and type hints to any new functions created

[rouille]

Thanks