content/pandas: Basic revision, no major changes.

[rkdarst]

• Closes: #135 (convert column back to years)
• Various other minor clarity changes, for example:
  • typo fixing
  • more explanation
  • add some df to show how we see values as we work.
  • etc.
• All code verified to work.
• Review: probably can be automerge

[bast]

Thanks!