alignment with ropensci statistical software standards

[s3alfisc]

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

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

[s3alfisc]

Fix all pkgcheck errors and warnings reported here: ropenscilabs/statistical-software-review#24.

[s3alfisc]

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

All things goodpractice:

• library() and require() calls - maybe in tests and examples (?)
• semicolons (?)
• replace sapply by vapply
• 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