Integrate R scripts

[khaeru]

This PR adds the two methods of invoking R code discussed in #40, and closes that issue.

Some things to be decided:

1. What kinds of conversions (see rpy2 docs) should be provided by default for R code?
2. What environment variables, utility code, etc. do standalone R scripts need to use from message-ix-models & co.?

How to review

TBA.

PR checklist

• Add CLI/helper for Rscript for invoking R scripts in a standard way.
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

Merging #58 (f85c3d5) into main (3ad3890) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #58   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           34        36    +2     
  Lines         1726      1752   +26     
=========================================
+ Hits          1726      1752   +26     

Impacted Files	Coverage Δ
...ssage-ix-models/message_ix_models/util/__init__.py	100.00% <0.00%> (ø)
message-ix-models/message_ix_models/util/r.py	100.00% <0.00%> (ø)
...e-ix-models/message_ix_models/tests/util/test_r.py	100.00% <0.00%> (ø)