feat: updated PSU logic (WIP)

[josephineweidner]

Builds on #91 (wherein PSU value calculated for years > 2020 were incorrect).

Current status of this branch is that it's WIP because

• Needs code cleanup (see TODO comments in code)
• PSU value is correct for latter years but now incorrect for earlier years (2020 is the cutoff I believe but would double check)

Twist thread with context: https://twist.com/a/133876/ch/695164/t/5973378/