local codecov tests

Parent=code coverage.

I do not see how to get local code coverage analysis working. I've spend a few hours doing web searches and have not found any advice that helps. I also made an issue on the covr website but I certainly cannot expect the covr author to figure this out for me, since I'm pretty sure it is yet another funky OSX thing.

Below are some things I've tried (in RStudio):

• add -ftest-coverage to the R build options. (get to the build options from menu Build/Configure Build Tools... then box Build and Reload.
• add -fprofile-arcs to the R build options
• try both of the above, after brew install gcovr and also after brew uninstall gcovr