Change to only retrieve CAISO fuel prices for Western Interconnect

[ktehranchi]

Checklist

• [X] I am using the current master branch
• [X] I am running on an up-to-date pypsa-usa environment. Update via conda env update -f envs/environment.yaml

The Issue

Per conversation with @casinocullen : Currently we download CAISO/WECC fuel prices regardless of interconnect. Change to be true if using only western or usa

Steps To Reproduce

No response

Expected Behavior

No response

Error Message

No response

Anything else?

No response