Updates to improve input demo documentation

[abbylewis]

Description

Updated the PEPRMT demos, including:

• More text explanation, especially in the first two demos
• Added year as a PEcAn standard variable (was listed as a TODO)
• New visualizations in the second demo so users see that it's working
  @infotroph not huge updates, but I think this should be helpful for CARB/etc to understand what's happening in the demos

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• My name is in the list of CITATION.cff
• I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• I have updated the CHANGELOG.md.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[infotroph]

Thanks! I agree this is a big improvement. I left a few suggestions throughout, mostly with wording suggestions you can modify or reject freely.

[infotroph]

At this point you've modified the source demo enough that it's probably appropriate to list yourself as the sole author and credit previous work with wording something like "This demo is patterned after PEcAn demo 1 (link here), which was written by Aritra Dey and David LeBauer". @AritraDey-Dev and @dlebauer does that make sense to y'all?

[infotroph]

Same suggestion to use ud_convert here as above