Merge R downscaling codes

[khaeru]

• [ ] Add command-line entry points for new code/methods (@khaeru)
• [ ] update the top-level and R READMEs
  • [ ] description of the downscaling codes (@pkyle)
  • [ ] better description of code/data separation and the need for item_config.yaml (@khaeru)
  • anything else?
• [ ] Add tests for new code
  • [X] Check: is there an R equivalent to the Python tools for measuring which lines of code are/are not covered by tests? (@khaeru)
  • [ ] Add coverage check using https://github.com/r-lib/covr.
• [ ] Check compliance with the Hadley R book instructions on release (@khaeru)

Note that:

• We'll release from the master branch after this is merged.
• We won't target 100% test coverage for this release, but should.

[khaeru]

Given the time elapsed, I'm going to merge this although the TODOs aren't complete.

IIASA's ixmp uses reticulate to provide an R interface to code written in Python. That might be a more sustainable track for us than maintaining two parallel codebases.

[pkyle]

Ahh shoot, I've got some revisions that I never pushed up to the branch b/c I had assumed it was dead! Oh well. I was thinking about making the downscaling scripts a stand-alone package; right now the scripts only work within this repo that you've built. What do you think?

[khaeru]

I think you can push the branch again and I can re-merge! Please try!

Yeah, that could be one way to go. Smaller, atomic pieces, that sort of idea.

[pkyle]

OK I think that worked--looks like it just created a new branch (gpk-branch)