alignment with ropensci statistical software standards

[s3alfisc]

For the standards, see here: https://stats-devguide.ropensci.org/index.html

• [ ] check with pkgcheck
• [x] check thatautotest::autotest() returns NULL
• [x] alignment with ssr package: regression standards

[s3alfisc]

Fix all pkgcheck errors and warnings reported here: https://github.com/ropenscilabs/statistical-software-review/issues/24.

[s3alfisc]

• [x] contributing file
• [x] add examples
• [ ] duplicate function names
• [ ] package coverage
• [ ] r cmd check

All things goodpractice:

• [ ] library() and require() calls - maybe in tests and examples (?)
• [ ] semicolons (?)
• [x] replace sapply by vapply
• [x] enforce 80 character lines
• [ ] rename functions pval, model_matrix
• [ ] cyclocomplexity

Other things:

• [ ] add check if Julia is installed, else don't run tests
• [ ] add tests with 'hard coded' values against Julia
• [ ] review technical vignette
• [ ] add vignette on how to create tables with modelsummary
• [ ] document undocumented functions