R package setup

[pdiakumis]

Setting up CPSR as an R package. Basically moved over the pcgrr/R/cpsr.R script from the PCGR repo here. Passes devtools::check(), just need to export the log4r_* functions from pcgrr dev.

TODO

• [x] documentation (pkgdown? keep readthedocs?)
• [x] conda recipe
• [x] GitHub Actions
  • [x] conda build
  • [x] pkgdown/readthedocs deployment

[pdiakumis]

@sigven this is ready for a quick review if you wish. I'll deal with the docs in a separate PR.

[sigven]

• looks good, Peter!