Add continuous integration

[GreatEmerald]

Travis CI can be used (at least as a starting point). It should be enabled for the repo by making a travis.yml file.

[arnikz]

see e.g. testthat R package for writing unit tests (#28)

[GreatEmerald]

Apparently Travis CI automatically picks those tests up, that's handy! It also should be able to run R CMD check by default as well, which in turn includes running the "examples" unit tests (but Codecov would probably not know about those).
Travis R documentation: https://docs.travis-ci.com/user/languages/r/

[arnikz]

My suggestion would be to follow this guide with (R) best practices. Please have a look at one of our R projects here (and the corresponding repo).

For different OSes we need to config Travis-CI/Circle-CI/AppVeyor:

• Linux
• OSX
• Windows

[GreatEmerald]

Solved by using GitHub Actions instead. Currently it's running a basic R CMD check, later on will make more extensive checks.