Feature/GitHub actions

[eddelbuettel]

Turns on CI at GitHub with coverage identical to what we had before, and using the same (Docker-based) setup

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[codecov-io]

Codecov Report

Merging #1128 (916574d) into master (a72a27c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1128   +/-   ##
=======================================
  Coverage   95.55%   95.55%           
=======================================
  Files          64       64           
  Lines        2789     2789           
=======================================
  Hits         2665     2665           
  Misses        124      124