Bump Roxygen to 7.3.1

[infotroph]

This is the current CRAN version of roxygen2 as of today, though as always we'll fall behind again before long.

All PEcAn developers will need to update roxygen2 on their machines, e.g. via remotes::install_version("roxygen2", "7.3.1"). make will refuse to run until you do this.

For posterity, here's how to bump Roxygen next time it's needed:

• Edit the Makefile to change EXPECTED_ROXYGEN_VERSION := 7.3.1 to EXPECTED_ROXYGEN_VERSION := [new version]
• Run make clean && make document to be sure Roxygen has run on all packages
• Check the console output for warnings from Roxygen, fix as needed
• run ./scripts/generate_dependencies.R to update the version of Roxygen recorded as a Docker dependency
• Grep the PEcAn folder for the string 7.3.1 to make sure no references were missed.
  • e.g. this time I found models/cable/DESCRIPTION -- Make currently skips cable, so I redocumented it manually)
• Review all changes: they should mostly just be updating the RoxygenNote: line of all the DESCRIPTION files.
• Once all looks good, commit and push and tell everyone to update their machines

[mdietze]

@infotroph that "instructions for posterity" would make a great addition to the book_source as part of the PR itself